## Summary
Implements the `git_worktree_create` tool with full validation and error handling per issue #4 acceptance criteria.
## Changes
- New `pi-worktree-create.ts` module with pure, testable functions (no pi SDK deps)
- `validateCreateWorktreeInput()` — validates path and branch name parameters
- `validatePathDoesNotExist()` — refuses if target path exists (injectable for testing)
- `isCurrentBranch()` — prevents creating worktree on current branch
- `buildWorktreeAddCommand()` — builds correct git command with `-b` flag support
- Integrated all helpers into `createCreateWorktreeTool()` in main extension
- 17 new tests covering all validation paths and edge cases (29 total, all passing)
## Testing
- [x] All 29 tests pass (`bun test .pi/extensions/*.test.ts`)
- [x] Full test suite run with no regressions
- [ ] Manual verification (requires pi runtime)
## Checklist
- [x] Self-reviewed the diff
- [x] Code follows project conventions
- [x] Descriptive naming (no single-character variables)
- [x] Documentation updated (JSDoc on all new functions)
Co-authored-by: David Kong <davkon@gmail.com>
Reviewed-on: #17
