New git_hooks.backend = "external" setting redirects Entire's hook
detection from .git/hooks/ to a user-managed external_dir (e.g. .husky,
common/git-hooks). In external mode Entire never writes to the user's
hook directory or to .git/hooks/ — it only checks for the marker
"Entire CLI hooks" in scripts the user maintains.

- settings: GitHooksSettings (backend + external_dir) with whole-block
  merge, repo-relative + no-".." validation, strict enum on backend
- strategy: GetGitHookDetectionDir branches detection by backend;
  InstallGitHook / RemoveGitHook / CheckAndWarnHookManagers short-circuit
  in external mode via shared externalGitHooksDir helper
- setup: verifyExternalGitHooksPrecondition gates every enable variant
  on external_dir existence and surfaces a 5-script instructional help
  message (FormatExternalDirMissingHelp, shared with doctor); status
  line uses PrintExternalHookStatusIfActive
- doctor: new checkExternalGitHooks surfaces ✓/✗ for external_dir,
  contributes to overall doctor exit code, stays silent in direct mode
- tests: settings (7) + hooks (6) + hook_managers (1) + doctor (3) +
  integration end-to-end Husky-shape (2)
- docs: new docs/architecture/external-git-hooks.md covering rationale,
  user contract, all 5 hook script templates, migration paths

Co-authored-by: Cursor <cursoragent@cursor.com>
Entire-Checkpoint: f184ee6ced2f