docs: adopt issue#-prefix naming for ADRs/PRDs to eliminate parallel-developer collisions (#3487)

* docs: adopt issue#-prefix naming for ADRs/PRDs (#3485)

The repo's sequential ADR/PRD numbering convention has produced
recurring collisions when developers compute "next number" locally
and ship in parallel — currently visible on disk as duplicate
docs/adr/0010-*.md and triplicate docs/adr/0011-*.md, plus a stack
of "resolve ADR conflict" commits in git history.

Replace the local-compute convention with issue#-prefix slug naming:

  docs/adr/<issue#>-<slug>.md     (new ADRs)
  docs/prd/<issue#>-<slug>.md     (new PRDs — directory introduced)

GitHub issue numbers are server-assigned and atomic, so the
reservation step the CONTRIBUTING.md issue-first rule already enforces
also produces the artifact ID. One issue = one ADR-or-PRD = one PR.
Same shape as the existing changeset random-name pattern (#2975) for
CHANGELOG.md fragments, applied to a different artifact class.

Migration policy: legacy ADRs 0001-* through 0011-* are preserved
as immutable historical record. The new convention applies only to
ADRs/PRDs created on or after this merge.

Files updated:
- docs/adr/README.md        — naming convention + legacy note + link
- docs/prd/README.md (new)  — seeds the new directory + same convention
- CONTRIBUTING.md           — new "Proposing an ADR or PRD" section
- docs/contributor-standards.md — formalize as contributor requirement

No code surface — docs-only.

Closes #3485

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* docs(changeset): add Changed fragment for ADR/PRD naming convention (#3487)

Per CONTRIBUTING.md "When unsure whether a change is user-facing, add
the fragment" — the contributor process IS user-facing for the
contributor user class. Drop the no-changelog opt-out, surface the
naming-convention change in the next CHANGELOG so contributors see
it before they hit it as a PR rejection.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* docs: address CodeRabbit findings on #3487

- CONTRIBUTING.md: rename heading to "Proposing an ADR or PRD" so its
  GitHub-anchor slug matches the #proposing-an-adr-or-prd link target
  used from docs/adr/README.md, docs/prd/README.md, and
  docs/contributor-standards.md (broken anchors)
- docs/adr/README.md, docs/prd/README.md, docs/contributor-standards.md:
  add `text` language tag to the new naming-convention fenced blocks
  to satisfy markdownlint MD040

Pre-existing untyped fences elsewhere in the touched files are left
alone per CONTRIBUTING.md "no drive-by formatting".

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

CONTRIBUTING.md

@@ -67,6 +67,30 @@ A feature adds something new — a new command, a new workflow, a new concept, a
 
 ---
 
+### 📐 Proposing an ADR or PRD
+
+An ADR (Architecture Decision Record) documents a significant architectural decision. A PRD (Product Requirements Document) captures the what and why of a feature before implementation. Both are governed by the same issue-first rule as everything else.
+
+**Process:**
+
+1. Open an issue of the appropriate type (enhancement for an ADR revisiting an existing area, feature for a new architectural surface, chore for policy/docs decisions). Fill it out completely.
+2. **Wait for maintainer approval.** A maintainer must label the issue `approved-enhancement`, `approved-feature`, or confirm the chore before any file is created.
+3. The GitHub-assigned issue number becomes your filename prefix. Create the file on a branch named after the issue:
+   - `docs/adr/<issue#>-<slug>.md` for ADRs
+   - `docs/prd/<issue#>-<slug>.md` for PRDs
+   - Branch: `docs/<issue#>-<slug>`
+4. Open a PR using the appropriate template and close the issue with `Closes #<issue#>` in the PR body.
+
+**One issue = one ADR-or-PRD = one PR.** Do not batch multiple decisions into one file or one PR.
+
+**Do not compute a "next number" locally.** Any PR that uses the legacy `NNNN-*` sequential pattern for a *new* ADR or PRD will be asked to rename the file to the `<issue#>-<slug>.md` format before merge.
+
+**Example:** Issue #3485 was opened, approved, and its number became the prefix: `docs/adr/3485-adr-prd-naming-convention.md` on branch `docs/3485-adr-prd-naming-convention`.
+
+**Rejection reasons:** Issue not approved before file was created, filename uses local-compute sequential number instead of issue#, multiple decisions bundled in one PR, file placed in wrong directory (`docs/adr/` vs `docs/prd/`).
+
+---
+
 ## The Issue-First Rule — No Exceptions
 
 > **No code before approval.**