Implement safe file-level Stage and Unstage - #55

Summary

• add file-level and grouped Stage and Unstage actions for the selected Worktree
• revalidate repository, Worktree generation, target state, locks, and stale baselines immediately before each mutation
• support literal NUL-delimited paths, Initial Commit Unstage, per-path Partial Success, and independent Worktree mutation lanes
• use bounded O(N) baseline inspection with O(1) per-target revalidation

Verification

• focused tests: 24 passed
• full test suite: 268 passed
• TypeScript typecheck
• ESLint
• Prettier
• production build
• git diff check

Closes #10