docs(contributing): add PR description template and convention

Codifies the PR body format the repo has converged on across its first
122 pull requests, which until now lived only as an implicit pattern
agents and contributors had to reverse-engineer from prior PRs.

- **`.github/PULL_REQUEST_TEMPLATE.md`** — `## Summary`, optional `##`
  sections, then `## Manual Verification` as a checkbox list. Gitea falls
  back to `.github/` when `.gitea/` is absent, and the GitHub mirror reads
  the same path, so one file covers both.
- **`CONTRIBUTING.md`** — new "Pull request descriptions" subsection under
  Branching & Pull Requests, documenting the same three-part structure.

Section names were chosen by frequency across all 122 PRs: `## Summary`
(86) and `## Manual Verification` (63) are the clear majority. The later
`## Manual Verification Checklist` (12, PRs 90-113), `## Test plan` (11)
and `## Verification` (3, PRs 121-122) variants are drift away from that
baseline, not a newer standard, so the template restores the dominant
form. Checkboxes ship unchecked but are meant to be checked before the
PR opens, matching the 585-to-12 ratio of `[x]` to `[ ]` in merged PRs.

CONTRIBUTING.md

@@ -64,6 +64,26 @@ assumes you *do* have push access (maintainers, regular contributors).
      approval before doing this** — there's no standing exception, no matter
      how trivial the change looks.
 
+### Pull request descriptions
+
+Fill in `.github/PULL_REQUEST_TEMPLATE.md` — Gitea and the GitHub mirror
+both pre-load it into the description box for you. Every PR carries, in
+this order:
+
+- **`## Summary`** — what changed and why, as a short paragraph, 2-5
+  bullets, or both. Name concrete paths and identifiers in backticks, and
+  explain the reasoning rather than restating the diff.
+- **Optional `##` sections** — add what the change actually needs
+  (`Root cause`, `Why`, `How it works`, `Behavior`, `Docs`, `Notes`,
+  `Scope note`, `Opinionated guard (C1-C6)`), and skip them entirely for a
+  straightforward change. A breaking change (title ending in `!` before
+  the colon) must include `## ⚠️ Breaking Change` with the migration path.
+- **`## Manual Verification`** — always last. Reproducible steps as a
+  checkbox list, each with the exact command and its expected result. Run
+  them before opening the PR and check the boxes; a box left unchecked
+  means that step was *not* verified, so say so explicitly rather than
+  leaving it silently blank.
+
 ## Commit Conventions
 
 Commit subjects follow [Conventional Commits](https://www.conventionalcommits.org/):