fix: hook resilience and worker lifecycle improvements (#957, #923, #984, #987, #1042)

Reduce timeouts to eliminate 10-30s startup delay when worker is dead
(common on WSL2 after hibernate). Add stale PID detection, graceful
error handling across all handlers, and error classification that
distinguishes worker unavailability from handler bugs.

- HEALTH_CHECK 30s→3s, new POST_SPAWN_WAIT (5s), PORT_IN_USE_WAIT (3s)
- isProcessAlive() with EPERM handling, cleanStalePidFile()
- getPluginVersion() try-catch for shutdown race (#1042)
- isWorkerUnavailableError: transport+5xx+429→exit 0, 4xx→exit 2
- No-op handler for unknown event types (#984)
- Wrap all handler fetch calls in try-catch for graceful degradation
- CLAUDE_MEM_HEALTH_TIMEOUT_MS env var override with validation

src/cli/handlers/index.ts

@@ -5,6 +5,7 @@
  */
 
 import type { EventHandler } from '../types.js';
+import { HOOK_EXIT_CODES } from '../../shared/hook-constants.js';
 import { contextHandler } from './context.js';
 import { sessionInitHandler } from './session-init.js';
 import { observationHandler } from './observation.js';
@@ -35,14 +36,22 @@ const handlers: Record<EventType, EventHandler> = {
 /**
  * Get the event handler for a given event type.
  *
+ * Returns a no-op handler for unknown event types instead of throwing (fix #984).
+ * Claude Code may send new event types that the plugin doesn't handle yet —
+ * throwing would surface as a BLOCKING_ERROR to the user.
+ *
  * @param eventType The type of event to handle
- * @returns The appropriate EventHandler
- * @throws Error if event type is not recognized
+ * @returns The appropriate EventHandler, or a no-op handler for unknown types
  */
-export function getEventHandler(eventType: EventType): EventHandler {
-  const handler = handlers[eventType];
+export function getEventHandler(eventType: string): EventHandler {
+  const handler = handlers[eventType as EventType];
   if (!handler) {
-    throw new Error(`Unknown event type: ${eventType}`);
+    console.error(`[claude-mem] Unknown event type: ${eventType}, returning no-op`);
+    return {
+      async execute() {
+        return { continue: true, suppressOutput: true, exitCode: HOOK_EXIT_CODES.SUCCESS };
+      }
+    };
   }
   return handler;
 }