feat: multi-branch release PRs

[decofe]

Support creating separate release PRs per branch, so patch releases on older versions can coexist with the main release train.

Example: foundry ships v1.6.0 from master, but also patches v1.5.x from a release/v1.5 branch. The action should create independent release PRs:

• changelog-release/master → v1.6.0
• changelog-release/release-v1.5 → v1.5.2

The action already accepts a branch input — this could be extended to derive the branch name from the trigger branch, e.g. changelog-release/${{ github.ref_name }}.

Changelog entries on each branch would be independent.