ci(prerelease): use committed release-notes, drop CI claude regen

CI runners don't have Claude Code installed, so the regen step in the
prerelease workflow died on 'claude CLI not found'. Mirror the stable
release model: generate prerelease-notes.md locally (claude -p), commit
it, and have CI just verify and reference the committed file.

docs/RELEASING.md

@@ -55,7 +55,10 @@
    `bun run build`
    When validating packaged updater output, confirm the platform build writes
    `*.yml` and `*.blockmap` files under `release/`.
-5. Commit the prerelease prep. Do not run `bun run changelog:build`.
+5. Commit the prerelease prep (package.json version bump + the generated
+   `release/prerelease-notes.md`). CI does not regenerate notes — it uses the
+   committed file — so review it before committing. Do not run
+   `bun run changelog:build`.
 6. Tag the commit: `git tag v<version>`.
 7. Push commit + tag.