v1.5.5

Add optional worktree setup step to dev-checkpoint (#4)

• Adds a new Step 9.5 to /dev-checkpoint that offers to set up a git worktree-based workflow on the first-ever checkpoint for a feature
• When accepted, creates a feature branch + worktree in a sibling directory, moves PRD files there, commits them, and updates checkpoint frontmatter
• Step is automatically skipped when not on main/master or when the feature branch already exists