Replace date-based auto feature branch names (feature/stacked-YYYYMMDD)
with semantically meaningful names derived from AI-generated commit
messages. The text generation prompt now returns a branch field alongside
subject and body, and a new suggestCommitAndBranch endpoint exposes this
to the web client for the default-branch checkout flow.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>