Adopt canonical planning-convention repo (consumer #1)

[lesnik512]

Replaces copy-paste sharing of the planning convention with a canonical source repo + an agent-applied instruction doc, and wires modern-di as its first consumer.

The mechanism

The convention (the index.py validator/index, _templates/, the Quick-path + Conventions prose) now lives in a personal, public canonical repo lesnik512/planning-convention (live, tag 1.0.0). Each consuming repo records the version it last applied in planning/.convention-version. To adopt or update, an agent reads the canonical repo's APPLY.md and applies the delta — verbatim copy for the clean files (index.py, _templates/), judgment-merge for the files that mix convention with repo-specific content (justfile, CLAUDE.md, the README portable block) — then verifies with just check-planning and opens a PR.

Chosen over Copier / a published package / pre-commit because the convention spans mixed files Copier can't merge, and an agent handles those intelligently with no new tooling or dependency. Personal (not org) repo so it can be consumed across several orgs.

This PR (modern-di side only)

• planning/.convention-version = 1.0.0.
• planning/README.md: the ## Conventions blockquote repointed from "copy this section" to the canonical source + APPLY.md update flow.
• planning/deferred.md: the sibling-rollout item now describes running APPLY.md against each sibling.
• The change bundle (design.md + plan.md) under planning/changes/2026-06-25.03-canonical-convention-repo/.

modern-di's index.py/_templates//prose already equal the canonical 1.0.0 (the canonical repo was seeded from them), so there's no file churn beyond the marker + pointer.

Verification

• just check-planning → planning: OK
• just test-ci → 232 passed, 100% coverage
• just lint-ci → clean
• just docs-build → OK

Sibling-repo rollout is tracked in planning/deferred.md.

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