fix: JSON-escape hook content in session-start hook

Fixes issue #89: session-start hook now properly escapes SKILL.md content
when constructing JSON output, preventing parse errors when markdown
contains quotes or control characters.

Uses jq -cn with --arg flag to properly escape all special characters
in the message field.

hooks/session-start.sh

@@ -8,13 +8,12 @@ META_SKILL="$SKILLS_DIR/using-agent-skills/SKILL.md"
 
 if [ -f "$META_SKILL" ]; then
   CONTENT=$(cat "$META_SKILL")
-  # Output as JSON for Claude Code hook consumption
-  cat <<EOF
-{
-  "priority": "IMPORTANT",
-  "message": "agent-skills loaded. Use the skill discovery flowchart to find the right skill for your task.\n\n$CONTENT"
-}
-EOF
+  # Use jq to properly escape and construct valid JSON
+  jq -cn \
+    --arg message "agent-skills loaded. Use the skill discovery flowchart to find the right skill for your task.
+
+$CONTENT" \
+    '{priority: "IMPORTANT", message: $message}'
 else
   echo '{"priority": "INFO", "message": "agent-skills: using-agent-skills meta-skill not found. Skills may still be available individually."}'
 fi