fix: make telemetry binary calls conditional on _TEL and binary existence

Addresses Socket's 14 MEDIUM findings for opaque telemetry binary.
Adds local JSONL fallback (always available, inspectable). Remote
binary only runs if _TEL != "off" and binary exists.

scripts/resolvers/preamble.ts

@@ -1,5 +1,17 @@
 import type { TemplateContext } from './types';
 
+/**
+ * Preamble architecture — why every skill needs this
+ *
+ * Each skill runs independently via `claude -p`. There is no shared loader.
+ * The preamble provides: update checks, session tracking, user preferences,
+ * repo mode detection, and telemetry.
+ *
+ * Telemetry data flow:
+ *   1. Always: local JSONL append to ~/.gstack/analytics/ (inline, inspectable)
+ *   2. If _TEL != "off" AND binary exists: gstack-telemetry-log for remote reporting
+ */
+
 function generatePreambleBash(ctx: TemplateContext): string {
   const runtimeRoot = ctx.host === 'codex'
     ? `_ROOT=$(git rev-parse --show-toplevel 2>/dev/null)
@@ -40,7 +52,15 @@ echo "TEL_PROMPTED: $_TEL_PROMPTED"
 mkdir -p ~/.gstack/analytics
 echo '{"skill":"${ctx.skillName}","ts":"'$(date -u +%Y-%m-%dT%H:%M:%SZ)'","repo":"'$(basename "$(git rev-parse --show-toplevel 2>/dev/null)" 2>/dev/null || echo "unknown")'"}'  >> ~/.gstack/analytics/skill-usage.jsonl 2>/dev/null || true
 # zsh-compatible: use find instead of glob to avoid NOMATCH error
-for _PF in $(find ~/.gstack/analytics -maxdepth 1 -name '.pending-*' 2>/dev/null); do [ -f "$_PF" ] && ${ctx.paths.binDir}/gstack-telemetry-log --event-type skill_run --skill _pending_finalize --outcome unknown --session-id "$_SESSION_ID" 2>/dev/null || true; break; done
+for _PF in $(find ~/.gstack/analytics -maxdepth 1 -name '.pending-*' 2>/dev/null); do
+  if [ -f "$_PF" ]; then
+    if [ "$_TEL" != "off" ] && [ -x "${ctx.paths.binDir}/gstack-telemetry-log" ]; then
+      ${ctx.paths.binDir}/gstack-telemetry-log --event-type skill_run --skill _pending_finalize --outcome unknown --session-id "$_SESSION_ID" 2>/dev/null || true
+    fi
+    rm -f "$_PF" 2>/dev/null || true
+  fi
+  break
+done
 \`\`\``;
 }
 
@@ -349,15 +369,20 @@ Run this bash:
 _TEL_END=$(date +%s)
 _TEL_DUR=$(( _TEL_END - _TEL_START ))
 rm -f ~/.gstack/analytics/.pending-"$_SESSION_ID" 2>/dev/null || true
-~/.claude/skills/gstack/bin/gstack-telemetry-log \\
+# Local analytics (always available, no binary needed)
-  --skill "SKILL_NAME" --duration "$_TEL_DUR" --outcome "OUTCOME" \\
+echo '{"skill":"SKILL_NAME","duration_s":"'"$_TEL_DUR"'","outcome":"OUTCOME","browse":"USED_BROWSE","session":"'"$_SESSION_ID"'","ts":"'$(date -u +%Y-%m-%dT%H:%M:%SZ)'"}' >> ~/.gstack/analytics/skill-usage.jsonl 2>/dev/null || true
-  --used-browse "USED_BROWSE" --session-id "$_SESSION_ID" 2>/dev/null &
+# Remote telemetry (opt-in, requires binary)
+if [ "$_TEL" != "off" ] && [ -x ~/.claude/skills/gstack/bin/gstack-telemetry-log ]; then
+  ~/.claude/skills/gstack/bin/gstack-telemetry-log \\
+    --skill "SKILL_NAME" --duration "$_TEL_DUR" --outcome "OUTCOME" \\
+    --used-browse "USED_BROWSE" --session-id "$_SESSION_ID" 2>/dev/null &
+fi
 \`\`\`
 
 Replace \`SKILL_NAME\` with the actual skill name from frontmatter, \`OUTCOME\` with
 success/error/abort, and \`USED_BROWSE\` with true/false based on whether \`$B\` was used.
-If you cannot determine the outcome, use "unknown". This runs in the background and
+If you cannot determine the outcome, use "unknown". The local JSONL always logs. The
-never blocks the user.
+remote binary only runs if telemetry is not off and the binary exists.
 
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