refactor: simplify nightly release notes workflow

[vitormattos]

Summary

Replaces the large inline Bash/JQ implementation in workflow-templates/nightly-release.yml with the reusable release-notes-from-pull-requests action merged in #174.

The workflow now only:

• resolves the lower Git tag boundary;
• calls the shared action with repository, branch and Git range;
• wraps the generated change list in the nightly release-note header/footer.

All pull-request association, branch preference, deduplication, commit fallback, Markdown sanitization and GitHub API handling live in the tested action.

Why

The previous workflow embedded business logic in YAML:

• commit enumeration;
• per-commit API requests;
• PR selection;
• PR deduplication;
• fallback handling;
• Markdown rendering.

That made the template harder to review, reuse and test. The new design keeps workflow YAML focused on orchestration.

Validation

The nightly template tests now assert that:

• the shared action is pinned to immutable merged SHA db491a51227ff4fd76b35b7a24ab7718aab6cbee;
• the nightly branch and Git range are passed to the action;
• the generated changes file is consumed;
• the old inline loop/JQ implementation is absent.

The action itself has focused unit tests added in #174.

Signed-off-by: Vitor Mattos vitor@php.rio