refactor(hooks): use sessionStart.skill instead of command hooks

- Replace SessionStart command hook with plugin.json's native
  sessionStart.skill field (like Superpowers' using-superpowers pattern)
- engineering-principles skill auto-loads at every session start
- Remove hooks/session-start, hooks/run-hook.cmd scripts
- Remove obsolete setup-hooks.sh (references deleted scripts)
- Empty hooks.json — hooks not needed, skill auto-load handles it

.claude-plugin/plugin.json

@@ -8,5 +8,7 @@
   },
   "keywords": ["best-practice", "clean-code", "engineering-guide", "programming-standards", "architecture"],
   "skills": "./skills/",
-  "hooks": "./hooks/hooks.json"
+  "sessionStart": {
+    "skill": "engineering-principles"
+  }
 }