feat(release): auto-promote [Unreleased] into [<version>] on release workflow run (#436)

Fixes the silent-sparse-release-notes failure mode that surfaced on
v0.9.5: the Release workflow used to do a literal
`extract-release-notes.mjs <version>` lookup with an `[Unreleased]`
fallback. The fallback only triggered when the `[<version>]` block
DIDN'T exist at all — and in practice maintainers sometimes had a
sparse `[<version>]` block pre-populated (e.g. one early fix
documented before the rest of the work landed). The workflow then
extracted that sparse block, ignoring the much-larger `[Unreleased]`
section above it. Result: the published v0.9.5 release notes were
missing the shared MCP daemon, the per-file staleness banner, the
Objective-C indexing, AND the Mixed iOS/RN/Expo bridging.

The fix is a new `scripts/prepare-release.mjs` step that runs at the
start of the workflow:

  Case A — `[<version>]` does not yet exist:
    Rename `[Unreleased]` → `[<version>] - <today>`. Add a fresh
    empty `[Unreleased]` above. The common path.

  Case B — `[<version>]` exists AND `[Unreleased]` has content:
    Merge `[Unreleased]`'s sub-sections (### Added / ### Fixed /
    ### Changed / ### Removed / ### Deprecated / ### Security) into
    the corresponding sub-sections of `[<version>]`. Unmatched
    sub-sections are appended. Then empty `[Unreleased]`.

  Case C — `[Unreleased]` is empty:
    No-op. Re-runs of the workflow are safe.

After the script runs, the workflow auto-commits + pushes the
CHANGELOG.md change back to main (with a `[skip ci]` tag in the
commit body) so future runs and human eyes both see the same
on-disk truth.

9 unit tests (`__tests__/prepare-release.test.ts`) cover all three
cases, idempotency, version-source precedence, and an
extract-release-notes.mjs integration check.

Workflow comment header rewritten to reflect the new flow.

Trigger reminder going forward: bump package.json. CHANGELOG entries
can live under `[Unreleased]` — the workflow takes care of moving
them.

937/939 existing tests pass (2 pre-existing skips); +9 new tests.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>