Add pull request template

.github/pull_request_template.md

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+<!--
+This template is a guide, not a gate. Use as much or as little as helps the
+reviewer understand your change. For straightforward PRs (dependency bumps,
+linting fixes, CI tweaks, typos) a one-liner description is perfectly fine
+-- delete the sections that don't add value.
+
+Rule of thumb: the more the change affects behavior, the more sections you
+should fill in.
+-->
+
+# Why
+
+<!-- Problem statement: what's broken/slow/confusing/missing today? (1-3 sentences) -->
+
+<!-- Goal: what outcome does this PR achieve? -->
+
+<!-- Non-goals: what this PR intentionally does NOT do (prevents scope creep) -->
+
+Closes <!-- #issue -->
+
+## What changed
+
+<!-- Group changes by intent, not by file. -->
+
+<!-- Behavioral changes: what users/systems will observe differently -->
+-
+
+<!-- Implementation notes: key design choices, tradeoffs, notable refactors -->
+
+<!-- What stayed the same: especially useful if you touched sensitive areas -->
+<!-- e.g., "No schema changes", "API contract unchanged", "Only affects branch X" -->
+
+<!-- If the diff is large, add a suggested review order:
+### Suggested review order
+1. Start with ...
+2. Then ...
+3. Tests are ...
+-->
+
+## How to review
+
+<!-- Key files/areas to focus on vs. mechanical/generated changes -->
+
+<!-- Risky or uncertain parts where you want extra scrutiny -->
+
+<!-- Alternatives considered (only if it affects future direction) -->
+
+## How to test
+
+<!-- Make it runnable and specific -->
+
+```bash
+# Commands to validate
+```
+
+<!-- Expected outputs, screenshots, or links to CI results -->
+
+## Impact & rollout
+
+<!-- Delete items that don't apply -->
+
+- **Backward compatibility:** <!-- any breaking changes or migration steps -->
+- **Performance:** <!-- implications, measurements if relevant -->
+- **Config/env changes:** <!-- new settings, feature flags, env vars -->
+- **Deployment notes:** <!-- "safe to deploy" vs "requires coordinated release" -->
+
+## Checklist
+
+- [ ] Tests added/updated
+- [ ] Changelog entry added (`uv run towncrier create ...`)
+- [ ] External docs updated (if user-facing or ops-facing change)
+- [ ] Internal .md docs updated (internal knowledge and AI code tools knowledge)