Add hooks and refactor skills to use resources pattern

Phase 1 of plugin-structure refactor:

- Add hooks/hooks.json for SessionStart automation
- Refactor gmail skill:
  - Extract inline scripts to scripts/check_unread.py, check_urgent.py, search.py
  - Add references/query-patterns.md for query documentation
  - Simplify SKILL.md to reference scripts instead of inline code
- Add gcal/scripts/agenda.py for direct calendar access
- Make all scripts executable

This follows the "Skill with Bundled Resources" pattern from
developing-claude-code-plugins best practices.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

hooks/hooks.json

@@ -0,0 +1,14 @@
+{
+  "hooks": {
+    "SessionStart": [
+      {
+        "hooks": [
+          {
+            "type": "command",
+            "command": "echo 'SessionStart:Callback hook success: Success'"
+          }
+        ]
+      }
+    ]
+  }
+}