feat(contract-validator): add SessionStart auto-validate hook (#229)

Implements smart SessionStart hook for plugin validation:
- Validates plugin contracts only when files change (smart mode)
- Caches file hashes to avoid redundant checks
- Non-blocking warnings for compatibility issues
- Configurable via CONTRACT_VALIDATOR_AUTO_CHECK

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

plugins/contract-validator/hooks/hooks.json

@@ -0,0 +1,10 @@
+{
+  "hooks": {
+    "SessionStart": [
+      {
+        "type": "command",
+        "command": "${CLAUDE_PLUGIN_ROOT}/hooks/auto-validate.sh"
+      }
+    ]
+  }
+}