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>

docs/contributor-standards.md

@@ -89,7 +89,22 @@ An ADR is optional (a comment in the relevant issue or PR is sufficient) when:
 
 ### Naming conventions
 
-`NNNN-<short-slug>.md` — four-digit zero-padded sequence number, followed by a kebab-case slug that names the Module or decision. Example: `0003-model-catalog-module.md`.
+**New ADRs and PRDs use issue#-prefix slug naming. This is a contributor requirement, not a suggestion.**
+
+```text
+docs/adr/<issue#>-<kebab-slug>.md    (new ADRs)
+docs/prd/<issue#>-<kebab-slug>.md    (new PRDs)
+```
+
+Example: `docs/adr/3485-adr-prd-naming-convention.md`.
+
+**Why:** GitHub issue numbers are server-assigned and atomic — the reservation mechanism already exists because the issue-first rule requires it. Promoting the issue# to the artifact ID eliminates the entire collision class that the `NNNN-*` local-compute scheme created (see the `0010-*` × 2 and `0011-*` × 3 duplicates on disk).
+
+**Migration policy:** Legacy ADRs `0001-*` through `0011-*` keep their numbers as immutable historical record. The new convention applies to all ADRs and PRDs created on or after the merge of the implementing PR (#3485). Do not renumber legacy files.
+
+For the end-to-end workflow — opening the issue, waiting for approval, creating the file, and submitting the PR — see **[CONTRIBUTING.md — "Proposing an ADR or PRD"](../CONTRIBUTING.md#proposing-an-adr-or-prd)**.
+
+The legacy four-digit scheme (`0003-model-catalog-module.md`) applies only to pre-existing files.
 
 ### Required sections
 
@@ -121,7 +136,7 @@ Reference sibling ADRs by filename, not by title prose: `see \`0001-dispatch-pol
 
 ### ADR README index
 
-`docs/adr/` does not currently maintain a separate `README.md` index. The canonical index is the table in this document (above). If an ADR is added, update this table in the same PR.
+`docs/adr/README.md` maintains the canonical index table and the naming convention documentation. The table in this document (above) covers accepted ADRs for contributor reference. If an ADR is added, update both tables in the same PR.
 
 ### Governance