feat(setup): persistent timeline Stop hook opt-out — timeline_stop_hook config gate (#2677)

--no-team is a one-shot teardown, so every later bare ./setup (including the
ones /gstack-upgrade runs) re-registered the timeline Stop hook with no way
to say 'never'. New gate mirrors the plan_tune_hooks pattern: flag
(--timeline-stop-hook/--no-timeline-stop-hook) > env
(GSTACK_TIMELINE_STOP_HOOK) > saved config (timeline_stop_hook) > default
yes. An explicit flag persists to config so the decision survives upgrades;
an explicit 'no' also removes a live registration (reconciliation), so the
opt-out works against installs registered by an older setup. --no-team
semantics unchanged (NO_TEAM_MODE is never initialized from config). Full
gstack-config surface: DEFAULTS entry, header docs, list/defaults
enumeration, warn-and-default validation.

Fixes #2677

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

bin/gstack-config

@@ -93,6 +93,14 @@ CONFIG_HEADER='# gstack configuration — edit freely, changes take effect on ne
 #                           # Set to true once the privacy gate has asked the user.
 #                           # Flip back to false to be re-prompted.
 #
+# ─── Timeline Stop hook ──────────────────────────────────────────────
+# timeline_stop_hook: yes   # Controls whether ./setup registers the timeline
+#                           #   Stop hook (closes dangling session entries).
+#                           #   yes — register on every setup (default)
+#                           #   no  — never register; setup also removes a
+#                           #         live registration (persistent opt-out;
+#                           #         --no-team stays a one-shot teardown, #2677)
+#
 # ─── Plan-tune hooks ─────────────────────────────────────────────────
 # plan_tune_hooks: prompt   # Controls whether ./setup installs the plan-tune
 #                           #   Claude Code hooks (PostToolUse capture +
@@ -148,6 +156,7 @@ lookup_default() {
     artifacts_sync_mode) echo "off" ;;
     artifacts_sync_mode_prompted) echo "false" ;;
     plan_tune_hooks) echo "prompt" ;; # prompt | yes | no — controls ./setup plan-tune hook install
+    timeline_stop_hook) echo "yes" ;; # yes | no — controls ./setup timeline Stop hook registration (#2677)
 
     redact_repo_visibility) echo "" ;; # empty → fall through to gh/glab detection
     redact_prepush_hook) echo "false" ;;
@@ -396,6 +405,10 @@ case "${1:-}" in
       echo "Warning: plan_tune_hooks '$VALUE' not recognized. Valid values: prompt, yes, no. Using prompt." >&2
       VALUE="prompt"
     fi
+    if [ "$KEY" = "timeline_stop_hook" ] && [ "$VALUE" != "yes" ] && [ "$VALUE" != "no" ]; then
+      echo "Warning: timeline_stop_hook '$VALUE' not recognized. Valid values: yes, no. Using yes." >&2
+      VALUE="yes"
+    fi
     # codex_reviews controls PAID Codex calls. Unlike the warn-and-default keys above,
     # an invalid value is REJECTED and the existing setting is left unchanged — a typo
     # must never silently flip the switch and turn paid Codex calls on or off.
@@ -441,7 +454,8 @@ case "${1:-}" in
     for KEY in proactive routing_declined telemetry auto_upgrade update_check \
                skill_prefix checkpoint_mode checkpoint_push explain_level \
                codex_reviews gstack_contributor skip_eng_review workspace_root \
-               artifacts_sync_mode artifacts_sync_mode_prompted plan_tune_hooks; do
+               artifacts_sync_mode artifacts_sync_mode_prompted plan_tune_hooks \
+               timeline_stop_hook; do
       VALUE=$(read_config_value "$KEY" || true)
       SOURCE="default"
       if [ -n "$VALUE" ]; then
@@ -457,7 +471,8 @@ case "${1:-}" in
     for KEY in proactive routing_declined telemetry auto_upgrade update_check \
                skill_prefix checkpoint_mode checkpoint_push explain_level \
                codex_reviews gstack_contributor skip_eng_review workspace_root \
-               artifacts_sync_mode artifacts_sync_mode_prompted plan_tune_hooks; do
+               artifacts_sync_mode artifacts_sync_mode_prompted plan_tune_hooks \
+               timeline_stop_hook; do
       printf '  %-24s %s\n' "$KEY:" "$(lookup_default "$KEY")"
     done
     ;;