feat(hooks): Stop hook closes dangling timeline entries — fail-open

The preamble writes event:'started' to the project timeline at every skill
start, but the matching 'completed' write lives in prose at the END of the
skill workflow — unenforceable. An interrupted session, a context blowout,
or an agent that simply stops leaked started > completed forever, and the
leak was unrepairable after the fact (observed live in #2553).

New hosts/claude/hooks/timeline-stop-hook (+ .ts, question-log-hook shim
pattern): on Claude Code's Stop event it appends event:'completed' with
outcome 'unknown' and source 'stop-hook' for every 'started' entry in the
project timeline that has no matching completion. setup registers it via
gstack-settings-hook add-event (Stop was already an accepted event) under
its own source tag, idempotently; --no-team and gstack-uninstall remove it.

FAIL-OPEN contract (F5), pinned by tests: ALWAYS exits 0 — corrupt
timeline (bad lines skipped individually, valid ones still repaired),
missing timeline, garbage/empty stdin, bun missing from PATH (the shim
'|| true's), and an over-cap timeline (10MB skip) all repair nothing and
block nothing; errors land in ~/.gstack/hook-errors.log best-effort. The
write path is append-only with a ~2s internal budget, and a second Stop is
a no-op (already-closed entries never re-close). Correlation is
project-scoped by design — the preamble's session id is shell-local, so a
concurrent same-project session's entry may close early as a traceable
source:'stop-hook' row rather than a silent leak; the header documents the
trade-off.

Fixes #2553

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

bin/gstack-uninstall

@@ -318,6 +318,10 @@ if [ -x "$SETTINGS_HOOK" ]; then
   if "$SETTINGS_HOOK" remove-source --source plan-tune-cathedral 2>/dev/null | grep -q "removed [1-9]"; then
     REMOVED+=("plan-tune cathedral hooks")
   fi
+  # Timeline Stop hook (#2553).
+  if "$SETTINGS_HOOK" remove-source --source gstack-timeline-stop 2>/dev/null | grep -q "removed [1-9]"; then
+    REMOVED+=("timeline Stop hook")
+  fi
 fi
 
 # ─── Remove global state ────────────────────────────────────