[release] Automate version bump and release notes from conventional PRs

[StatPan]

Summary

• Replace the current manual-only release preparation flow with an automated release PR flow driven by conventional commits and squash-merge titles.

Acceptance Criteria

• A GitHub Action opens or updates a release PR on pushes to main.
• The release PR updates version files and CHANGELOG.md automatically.
• PR validation enforces a conventional release signal so merged changes are actually releasable.
• Docs describe the new flow clearly and demote the old manual script to fallback-only usage.
• The setup is validated with a dry-run or real workflow execution against the repo config.

[StatPan]

Implementation is up in PR #13.\n\nValidation completed so far:\n- strict pytest gate passed\n- actionlint passed on all workflows\n- release-please manifest config was parsed successfully in dry-run\n- release-please release-pr dry-run computed a release PR from branch and showed the expected version/file updates\n\nNext step is normal review/merge of PR #13.

[StatPan]

Implementation is up in PR #13.

Validation completed so far:

• strict pytest gate passed
• actionlint passed on all workflows
• release-please manifest config was parsed successfully in dry-run
• release-please release-pr dry-run against branch issue-12-release-automation computed a release PR and showed the expected version/file updates

Next step is normal review/merge of PR #13.

[StatPan]

Follow-up hardening: removing the GitHub Actions Node 20 deprecation warning from by opting this workflow into the Node 24 runtime.

[StatPan]

Follow-up hardening: removing the GitHub Actions Node 20 deprecation warning from release-please.yml by opting this workflow into the Node 24 runtime.

[StatPan]

Release automation is now merged and verified end-to-end: release PR creation, auto-generated release notes, tag/release publication, reusable OS build matrix handoff, and the follow-up Node 24 runtime hardening in PR #15. Remaining annotation is upstream: googleapis/release-please-action@v4 still declares Node 20 even when GitHub forces it onto Node 24.