[hdamker]

What type of PR is this?

• cleanup

What this PR does / why we need it:

Adds the modules the act job will call, with tests against an in-memory GitHub client (release_automation/tests/fake_github.py). Nothing is wired yet; the next PR replaces the command jobs with act and snapshot.

• draft_release.py replaces Extract CHANGELOG release notes, Create Draft Release, Discard Snapshot, Delete Draft Release and both Cleanup review branch steps. Same CHANGELOG candidates, candidate-block stripping (full block or orphan marker), release-notes extraction, fallback body, -preserved rename.
• It serves both publish_mode values of the plan: create_draft() for draft (today's path) and publish_on_merge() for on_merge: CHANGELOG cleanup commit, release_date commit, published release and tag at that commit, prerelease flag from the release type, then the same post-publication steps as /publish-release. All policy values are still draft, so on_merge does not run yet.
• release_publisher.publish_draft() replaces the 97-line Python heredoc of publish-release; complete_publication() (reference tag, pointer branch, branch cleanup, warnings) is shared by both modes.
• issue_events.py: reopen a Release Issue closed during a release, with the issue_reopened comment.
• The CHANGELOG cleanup commit on the snapshot branch is retried twice (5 s, 10 s) on GitHub's transient ruleset timeout ("Rule was unable to be completed in 10 seconds"); other errors fail as before. See Retry logic for transient GitHub ruleset validation timeouts #155.
• repository-setup.md now says what happens without README release-info delimiters: the README update is skipped with a warning (the docs said the step fails).

Behaviour differences, all on failure paths:

• a missing snapshot branch on /discard-snapshot or /delete-draft reports "Snapshot branch … not found" instead of the API's "Reference does not exist"
• a failed CHANGELOG cleanup commit returns success=false with "CHANGELOG cleanup failed: …" (it was an unhandled step failure); no draft is created in both cases

GitHubClient gains create_release, delete_release, delete_tag, reopen_issue and get_branch_sha; bot_responder.post_comment() is the comment helper finalize already had.

Which issue(s) this PR fixes:

Part of #476

Special notes for reviewers:

Targets release-automation-refactor. No change on a running path except finalize calling the shared post_comment().

Changelog input

 release-note

Additional documentation

docs