* feat(gsd): add directory safeguards to prevent running in system/home paths
GSD previously had no protection against being launched from dangerous
directories like $HOME, /, /usr, or /etc. This adds layered validation:
- Blocked system paths (hard stop): /, /usr, /etc, /var, $HOME, tmpdir, etc.
- High entry count heuristic (>200 entries triggers confirmation dialog)
- Symlink resolution via realpathSync to prevent bypass
- Integrated at three chokepoints: projectRoot(), showSmartEntry(), bootstrapGsdDirectory()
Includes 19 tests covering all blocked categories, boundary conditions, and
the assertSafeDirectory throw/return behavior.
* fix: make directory safeguard tests cross-platform (Windows CI)
- Skip Unix-specific blocked path tests on Windows (/, /usr, /etc, etc.)
- Add Windows-specific blocked path tests (C:\, C:\Windows)
- Use platform-appropriate path separator in trailing slash test
- Fix root path normalization for Windows drive letters (C:\ not C:)
