fix(preamble): update_check:false gates the prose, not just the binary

Setting update_check:false stopped the update-check BINARY from running,
but every skill preamble still shipped the upgrade-handling instruction
prose unconditionally — burning tokens on instructions that could never
fire and confusing agents into probing for upgrades anyway. The resolver
now suppresses the upgrade-flow prose when the config disables checks.

Fixes #2001.

Contributed by @jc0d35 (PR #2022).

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

codex/SKILL.md

@@ -83,6 +83,8 @@ if [ "$_EXPLAIN_LEVEL" != "default" ] && [ "$_EXPLAIN_LEVEL" != "terse" ]; then
 echo "EXPLAIN_LEVEL: $_EXPLAIN_LEVEL"
 _QUESTION_TUNING=$(~/.claude/skills/gstack/bin/gstack-config get question_tuning 2>/dev/null || echo "false")
 echo "QUESTION_TUNING: $_QUESTION_TUNING"
+_UPDATE_CHECK=$(~/.claude/skills/gstack/bin/gstack-config get update_check 2>/dev/null || echo "true")
+echo "UPDATE_CHECK: $_UPDATE_CHECK"
 mkdir -p ~/.gstack/analytics
 if [ "$_TEL" != "off" ]; then
 echo '{"skill":"codex","ts":"'$(date -u +%Y-%m-%dT%H:%M:%SZ)'","repo":"'$(_repo=$(basename "$(git rev-parse --show-toplevel 2>/dev/null)" 2>/dev/null | tr -cd 'a-zA-Z0-9._-'); echo "${_repo:-unknown}")'"}'  >> ~/.gstack/analytics/skill-usage.jsonl 2>/dev/null || true
@@ -155,6 +157,8 @@ If `PROACTIVE` is `"false"`, do not auto-invoke or proactively suggest skills. I
 
 If `SKILL_PREFIX` is `"true"`, suggest/invoke `/gstack-*` names. Disk paths stay `~/.claude/skills/gstack/[skill-name]/SKILL.md`.
 
+If `UPDATE_CHECK` is `"false"`, skip the next two lines — the update-check binary emits nothing in that mode, so there is no `UPGRADE_AVAILABLE` / `JUST_UPGRADED` output to act on.
+
 If output shows `UPGRADE_AVAILABLE <old> <new>`: read `~/.claude/skills/gstack/gstack-upgrade/SKILL.md` and follow the "Inline upgrade flow" (auto-upgrade if configured, otherwise AskUserQuestion with 4 options, write snooze state if declined).
 
 If output shows `JUST_UPGRADED <from> <to>`: print "Running gstack v{to} (just updated!)". If `SPAWNED_SESSION` is true, skip feature discovery.