feat: learnings count in preamble output

Every skill now prints "LEARNINGS: N entries loaded" during preamble,
making the compounding loop visible to the user.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/resolvers/preamble.ts

@@ -63,6 +63,15 @@ for _PF in $(find ~/.gstack/analytics -maxdepth 1 -name '.pending-*' 2>/dev/null
   fi
   break
 done
+# Learnings count
+eval "$(${ctx.paths.binDir}/gstack-slug 2>/dev/null)" 2>/dev/null || true
+_LEARN_FILE="\${GSTACK_HOME:-$HOME/.gstack}/projects/\${SLUG:-unknown}/learnings.jsonl"
+if [ -f "$_LEARN_FILE" ]; then
+  _LEARN_COUNT=$(wc -l < "$_LEARN_FILE" 2>/dev/null | tr -d ' ')
+  echo "LEARNINGS: $_LEARN_COUNT entries loaded"
+else
+  echo "LEARNINGS: 0"
+fi
 \`\`\``;
 }