release-notes: rich single-source release action

[sotashimozono]

Standardises the release-notes system on the one you like (ClassicalMonteCarlo's build_release_notes.py) and fixes the three issues (doubled ## Changelog, oversized index, verbosity) by making it the single source.

actions/release-notes (composite, bundles build_release_notes.py): builds one clean ## What's Changed → category → per-PR <details> from each merged PR's Proposed Changes / Usage or Results sections (the org PR template from #6), then creates/updates the Release. Caller:

- uses: actions/checkout@v4
  with: { fetch-depth: 0 }
- uses: QAtlasHub/.github/actions/release-notes@main

The three issues came from LatticeCore/Lattice2D letting release-drafter-draft + TagBot-default-changelog both write the release (3 overlapping PR lists). The clean setup (documented in the README):

• TagBot changelog: false — tag/release shell only.
• release-drafter = version-resolver + draft only; this action overwrites the body.
• PR template (templates: org-default issue + PR templates #6) supplies the two extracted sections.

PR template ⇄ this action are one coupled system — adopting both gives the clean rich notes org-wide.