fix: add event type enum to hooks schema and avoid shared RegExp state

- hooks.schema.json: add enum constraint for hook event types
  (PreToolUse, PostToolUse, PreCompact, SessionStart, SessionEnd,
  Stop, Notification, SubagentStop) — enables IDE autocompletion
  and compile-time validation
- utils.js countInFile: always create fresh RegExp to avoid shared
  lastIndex state when reusing global regex instances
- README: update AgentShield stats (751 tests, 73 rules)

schemas/hooks.schema.json

@@ -11,7 +11,18 @@
       ],
       "properties": {
         "type": {
-          "type": "string"
+          "type": "string",
+          "enum": [
+            "PreToolUse",
+            "PostToolUse",
+            "PreCompact",
+            "SessionStart",
+            "SessionEnd",
+            "Stop",
+            "Notification",
+            "SubagentStop"
+          ],
+          "description": "Hook event type that triggers this hook"
         },
         "command": {
           "oneOf": [