Polish changelog fragments with claude -p at release time

- Replace `renderGroupedChanges` with `polishFragmentsWithClaude` that pipes fragments through `claude -p --model sonnet` to merge related items, drop housekeeping noise, and produce user-facing release notes
- Internal fragments kept in CHANGELOG.md under a `<details>` collapse; dropped from GitHub release notes entirely
- CI no longer auto-runs `changelog:build` on tag-based releases — fails fast with a clear error if `changes/*.md` fragments are still pending; build locally and commit before tagging
- Add `runClaude` dep-injection seam to test surface; add failure-mode coverage (missing binary, empty output, missing headers, missing `<details>` wrapper)
- Delete implemented design doc; update `changes/README.md` and `docs/RELEASING.md` with claude CLI prerequisite and new workflow

.github/workflows/release.yml

@@ -351,12 +351,11 @@ jobs:
         id: version
         run: echo "VERSION=${GITHUB_REF#refs/tags/}" >> "$GITHUB_OUTPUT"
 
-      - name: Build changelog artifacts for release
+      - name: Guard against pending changelog fragments
         run: |
           if find changes -maxdepth 1 -name '*.md' -not -name README.md -print -quit | grep -q .; then
-            bun run changelog:build --version "${{ steps.version.outputs.VERSION }}"
-          else
-            echo "No pending changelog fragments found."
+            echo "::error::Pending changelog fragments detected. Run 'bun run changelog:build --version ${{ steps.version.outputs.VERSION }}' locally and commit the polished CHANGELOG.md before tagging. CI no longer auto-builds the changelog because the polish step requires the local 'claude' CLI."
+            exit 1
           fi
 
       - name: Verify changelog is ready for tagged release