chore(devops): add stale worktree cleanup process

[mrveiss]

Problem

During v0.2.0 release prep, 14 git worktrees were found lingering under .worktrees/, including 5 for already-merged PRs and closed issues. No automated or documented cleanup process exists.

Impact

• Disk space waste (each worktree is a full repo copy)
• Branch deletion fails when worktree holds the branch
• Confusion about which worktrees represent active work

Discovered During

v0.2.0 release — had to manually remove worktrees before merging PRs.

Suggested Fix

• Add a cleanup script or git hook that removes worktrees whose branches are merged/deleted
• Document worktree lifecycle in CLAUDE.md or developer guide
• Consider a pre-release checklist step: "prune stale worktrees"

[mrveiss]

Merged in PR #2478. Added scripts/cleanup-worktrees.sh with --dry-run support.