Fix create-agent git option availability

[selfcontained]

Summary

• constrain the Create Agent worktree and new-branch checkbox props by current repo availability
• preserve saved option state instead of resetting it during cwd transitions
• tie path validation to the exact cwd and keep the submitted payload aligned with visible availability
• add focused coverage for repo, non-repo, and worktree-disabled states

Validation

• pnpm run check
• pnpm run finalize:web
• focused web tests: 35 passed
• pnpm run test:e2e: 195 passed, 13 skipped
• frontend UX persona review: approved with no feedback items (review #1105)
• Playwright validation on desktop and 390px mobile