chore(planning): adopt decisions/ ADRs

CLAUDE.md

@@ -11,13 +11,14 @@ Guidance for AI agents (Claude Code, etc.) working in this repository.
 - [`architecture/`](architecture/) (repo root) — the per-capability living truth (overview, client, middleware, decoders, errors, resilience, optional extras, testing); the promotion target on every ship. **Read the relevant file before changing that capability.**
 - [`planning/README.md`](planning/README.md) — the planning conventions (two axes, change bundles, three lanes, frontmatter) + the change Index.
 - [`planning/changes/<YYYY-MM-DD.NN-slug>/`](planning/changes/) — per-change bundles (`design.md` + `plan.md`, or `change.md` for the lightweight lane).
+- [`planning/decisions/<YYYY-MM-DD>-<slug>.md`](planning/decisions/) — design decisions taken (esp. options rejected with a load-bearing reason), each with a Revisit trigger; listed by `just index`.
 - [`planning/audits/`](planning/audits/) — findings reports + `scripts/` tooling.
 - [`planning/retros/`](planning/retros/) — retrospectives.
 - [`planning/releases/`](planning/releases/) — per-version release notes (also published on GitHub Releases).
 - [`planning/deferred.md`](planning/deferred.md) — review-surfaced, not-yet-actionable items.
 - [`planning/_templates/`](planning/_templates/) — design/plan/change templates.
 
-**Per-feature workflow:** brainstorming → `design.md` in `planning/changes/<id>/` → writing-plans → `plan.md` in the same bundle → executing-plans (or subagent-driven-development) → requesting-code-review → finishing-a-development-branch. On ship, promote the conclusions into the affected `architecture/<capability>.md` by hand and set `status: shipped` + `pr` + `outcome` in the implementing PR — there is no folder move. The change listing is generated: run `just index`. Topic slugs are kebab-case descriptions (`msgspec-decoder-adapter`), not story IDs.
+**Per-feature workflow:** brainstorming → `design.md` in `planning/changes/<id>/` → writing-plans → `plan.md` in the same bundle → executing-plans (or subagent-driven-development) → requesting-code-review → finishing-a-development-branch. On ship, promote the conclusions into the affected `architecture/<capability>.md` by hand and set `status: shipped` + `pr` + `outcome` in the implementing PR — there is no folder move. The change listing is generated: run `just index`. A design decision taken without a code change — especially a candidate rejected with a load-bearing reason — is recorded as `planning/decisions/YYYY-MM-DD-<slug>.md` (the `decision.md` template, frontmatter `status: accepted|superseded`), each with a **Revisit trigger** so future reviews don't re-litigate it; listed by `just index`. Topic slugs are kebab-case descriptions (`msgspec-decoder-adapter`), not story IDs.
 
 ## Commands
 

planning/README.md

@@ -53,6 +53,9 @@ into `design.md` + `plan.md`.
 - **`design.md`** — the spec: the *thinking* (why, design, trade-offs, scope).
 - **`plan.md`** — the plan: the *sequencing* (the executor's task checklist).
 - **`change.md`** — both, condensed, for the lightweight lane.
+- **`decisions/<YYYY-MM-DD>-<slug>.md`** — one file per design decision taken
+  (especially options *rejected*), each with a revisit trigger, so reviews don't
+  re-litigate them; listed by `just index`.
 - **`releases/<semver>.md`** — per-release user-facing notes.
 - **`audits/<date>-<slug>.md`** — findings from a code/docs/bug-hunt sweep;
   spawns fix changes.
@@ -65,22 +68,27 @@ Templates live in [`_templates/`](_templates/).
 
 `design.md` / `change.md`: `status` (draft|approved|shipped|superseded),
 `date`, `slug`, `summary` (single line), `supersedes`, `superseded_by`, `pr`,
-`outcome`. `plan.md`: `status`, `date`, `slug`, `spec`, `pr`. Files in
-`architecture/` carry **no** frontmatter — living prose, dated by git.
+`outcome`. `plan.md`: `status`, `date`, `slug`, `spec`, `pr`.
+`decisions/*.md`: `status` (accepted|superseded), `date`, `slug`, `summary`,
+`supersedes`, `superseded_by`, `pr`.
+Files in `architecture/` carry **no** frontmatter — living prose, dated by git.
 
 ## Index
 
-The change listing is **generated**, not maintained — run `just index` to
-print it (grouped by `status`: In progress / Shipped / Superseded). The
-frontmatter in each bundle is the single source of truth; there is no
-committed copy to drift.
+The listing is **generated**, not maintained — run `just index` to print it:
+changes grouped by `status` (In progress / Shipped / Superseded), then
+decisions newest-first. The frontmatter in each bundle / decision file is the
+single source of truth; there is no committed copy to drift.
 
 ## Other
 
 - **[`architecture/`](../architecture/)** at the repo root — the living
   per-capability truth (overview, client, middleware, decoders, errors,
   resilience, optional extras, testing). This is the promotion target on
   every ship.
+- **[decisions/](decisions/)** — design decisions taken (and alternatives
+  rejected), each with a revisit trigger, so reviews don't re-litigate them;
+  indexed by `just index`.
 - **[audits/](audits/)** — findings reports (deep audit + delta audits) and
   their `scripts/` tooling.
 - **[deferred.md](deferred.md)** — real-but-unscheduled items with revisit

planning/_templates/decision.md

@@ -0,0 +1,26 @@
+---
+status: accepted        # accepted | superseded
+date: YYYY-MM-DD
+slug: my-decision
+summary: One line — shown in `just index`.
+supersedes: null
+superseded_by: null
+pr: null                # PR/commit where the decision was made or recorded
+---
+
+# One-line capitalized title
+
+**Decision:** What was decided, in a sentence.
+
+## Context
+
+Why this came up; the options that were on the table.
+
+## Decision & rationale
+
+The call and why — including why the alternatives were rejected. Enough that a
+future explorer doesn't re-litigate it.
+
+## Revisit trigger
+
+The concrete signal that should reopen this decision.

planning/index.py

@@ -1,18 +1,20 @@
 # ruff: noqa: INP001, D212  # planning/ is not a Python package; D212/D213 conflict differs from faststream-outbox
 """
-Generate the planning change index from bundle frontmatter.
+Generate the planning index from frontmatter.
 
-Run via ``just index``. Globs ``planning/changes/*/``, reads each bundle's
-``design.md`` (falling back to ``change.md``) frontmatter, and prints a
-Markdown listing grouped by lifecycle status to stdout. Never writes a file:
-the listing is a query over the bundles, not a committed artifact.
+Run via ``just index``. Globs ``planning/changes/*/`` (each bundle's ``design.md``,
+falling back to ``change.md``) and ``planning/decisions/*.md``, reads their
+frontmatter, and prints a Markdown listing to stdout — changes grouped by lifecycle
+status, then decisions newest-first. Never writes a file: the listing is a query over
+the files, not a committed artifact.
 """
 
 import pathlib
 import sys
 
 
 CHANGES_DIR = pathlib.Path(__file__).parent / "changes"
+DECISIONS_DIR = pathlib.Path(__file__).parent / "decisions"
 GROUPS: tuple[tuple[str, tuple[str, ...]], ...] = (
     ("In progress", ("draft", "approved")),
     ("Shipped", ("shipped",)),
@@ -55,8 +57,23 @@ def load_bundles() -> list[dict[str, str]]:
     return bundles
 
 
+def load_decisions() -> list[dict[str, str]]:
+    """Read frontmatter from every decision file under ``DECISIONS_DIR``."""
+    decisions: list[dict[str, str]] = []
+    if not DECISIONS_DIR.is_dir():
+        return decisions
+    for path in sorted(DECISIONS_DIR.glob("*.md")):
+        if path.name == "README.md" or path.name.startswith("_"):
+            continue
+        fields = parse_frontmatter(path.read_text(encoding="utf-8"))
+        fields["path"] = f"decisions/{path.name}"
+        fields["name"] = path.stem
+        decisions.append(fields)
+    return decisions
+
+
 def format_row(bundle: dict[str, str]) -> str:
-    """Render one bundle as a Markdown list item."""
+    """Render one bundle or decision as a Markdown list item."""
     slug = bundle.get("slug", "?")
     path = bundle.get("path", "")
     pr = bundle.get("pr") or "—"
@@ -70,24 +87,28 @@ def format_row(bundle: dict[str, str]) -> str:
     return line
 
 
-def render(bundles: list[dict[str, str]]) -> str:
-    """Render the full grouped Markdown listing."""
-    out = ["# Change index", "", "_Generated by `just index` — do not edit._", ""]
+def render(bundles: list[dict[str, str]], decisions: list[dict[str, str]]) -> str:
+    """Render the full Markdown listing: changes by status, then decisions."""
+    out = ["# Planning index", "", "_Generated by `just index` — do not edit._", "", "## Changes", ""]
     for title, statuses in GROUPS:
-        out += [f"## {title}", ""]
+        out += [f"### {title}", ""]
         rows = sorted(
             (b for b in bundles if b.get("status") in statuses),
             key=lambda b: b.get("name", ""),
             reverse=True,
         )
         out += [format_row(b) for b in rows] if rows else ["_None._"]
         out.append("")
+    out += ["## Decisions", ""]
+    decision_rows = sorted(decisions, key=lambda d: d.get("name", ""), reverse=True)
+    out += [format_row(d) for d in decision_rows] if decision_rows else ["_None._"]
+    out.append("")
     return "\n".join(out).rstrip() + "\n"
 
 
 def main() -> int:
     """Print the listing to stdout."""
-    sys.stdout.write(render(load_bundles()))
+    sys.stdout.write(render(load_bundles(), load_decisions()))
     return 0