fix: red-team findings — verify-gate identity, single quoting authority, Windows paths

Red-team pass over the hardened diff (several findings empirically verified
by the reviewer before reporting):

- KNOWN_HOOKS gains the sixth identity: gstack-verify-gate (README-documented
  opt-in Stop hook). A tag-stripped verify-gate entry previously survived
  prune-stale --all and errored at the end of EVERY turn after uninstall
  deleted the install root — the exact phantom-hook class this branch fixes.
  Uninstall also sweeps its tagged form.
- add-event is now the single quoting authority: every registered command is
  normalized through the same gsQuoteCmd/gsStripWrap round-trip the healer
  uses. Pre-fix, only SessionStart got caller-side quoting — a spaced/metachar
  canonical root registered broken plan-tune/AUQ/timeline hooks that the very
  next heal rewrote (the codebase disagreed with its own registrations).
- Windows: MSYS-form paths (/c/Users/...) are drive-translated for fs checks
  only (gsWinPath) — native bun resolved them drive-relative, so the heal
  judged every LIVE Windows hook dead and pruned it. The three AskUserQuestion
  hooks and the Stop hook now also get the mandatory 'bash ' prefix on
  Windows (previously only SessionStart did; extensionless bash shims
  otherwise hit the file-association dialog).
- CANONICAL_GSTACK_ROOT falls back to $HOME/.claude/skills/gstack when a
  CLAUDE_CONFIG_DIR-derived root was never installed (the installer hardcodes
  the home path — split-brain left such users permanently hookless).
- prune-stale preserves foreign entries that STARTED empty (they were
  silently deleted, uncounted, on every heal).
- The timeline Stop registration and its list-sources guard join the
  zero-silent-mutations contract (stderr attached).

Tests: verify-gate tag-stripped heal+sweep, started-empty preservation,
add-event quoting-authority round-trip.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bin/gstack-uninstall

@@ -153,6 +153,10 @@ if [ -x "$SETTINGS_HOOK" ]; then
   if "$SETTINGS_HOOK" remove-source --source gstack-timeline-stop | grep -q "removed [1-9]"; then
     REMOVED+=("timeline Stop hook")
   fi
+  # Verification stop hook (opt-in via README; user-registered, ours to sweep).
+  if "$SETTINGS_HOOK" remove-source --source verify-gate | grep -q "removed [1-9]"; then
+    REMOVED+=("verification Stop hook")
+  fi
   # Identity sweep for untagged strays (Claude Code strips _gstack_source
   # tags; pre-v1.67 setups baked worktree paths). Removes every gstack-owned
   # hook item, live or dead — the binaries they point at are being deleted.