Honor Registrator branch when creating releases

[arnavk23]

• Extract release branch from registry PR body (Registrator [branch=]) and prioritize it over config/default when creating releases.
• Thread version through release-branch helpers so per-version branch selection works.
• Add tests for branch parsing/priority and update release tests to avoid network calls.
• Fixes Look up release branch in commit comment #200

[Copilot]

Pull request overview

This PR enhances TagBot to honor the branch specification from Registrator when creating releases. The implementation extracts the branch information from the registry PR body and prioritizes it over the configured or default branch.

Key changes:

• Converts _release_branch from a property to a method that accepts a version parameter, enabling per-version branch selection
• Adds _branch_from_registry_pr() method to parse branch information from registry PR body using regex
• Updates all call sites to thread version information through the release-branch helpers

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
tagbot/action/repo.py	Implements branch extraction from registry PR body, converts _release_branch to version-aware method, and updates all dependent methods
test/action/test_repo.py	Adds comprehensive tests for branch parsing logic and updates existing tests to pass version parameters

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[arnavk23]

@IanButterworth whenever you are free, please take a look. Thanks!