[fix] Delete branches merged into selected base (#16)

[ThreeLight2000]

Fixes #16

Summary

• delete eligible local branches with the selected merge base as the safety boundary
• keep protecting the current and base branches
• add a regression test for a branch merged into main while HEAD is elsewhere
• remove obsolete type:* label enforcement from the branch-name workflow
• upgrade actions/github-script to v8 for the Node 24 runner

Root cause

The cleanup candidate list is computed with git for-each-ref --merged <base>, but git branch -d performs its safety check against the current HEAD when no upstream is configured. A branch can therefore be safely merged into the selected base yet fail deletion from another branch. The fix uses force deletion only after the candidate has passed the selected-base merge check.

Verification

• pnpm verify
• node --test --test-reporter=dot scripts/cleanup-merged.test.mjs (5 tests)
• git diff --check

Scope

Repository tooling and pull request validation only; no client or server runtime behavior changes.