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/services/worker-service.ts

@@ -14,6 +14,7 @@ import { existsSync, writeFileSync, unlinkSync, statSync } from 'fs';
 import { Client } from '@modelcontextprotocol/sdk/client/index.js';
 import { StdioClientTransport } from '@modelcontextprotocol/sdk/client/stdio.js';
 import { getWorkerPort, getWorkerHost } from '../shared/worker-utils.js';
+import { HOOK_TIMEOUTS } from '../shared/hook-constants.js';
 import { SettingsDefaultsManager } from '../shared/SettingsDefaultsManager.js';
 import { logger } from '../utils/logger.js';
 
@@ -66,6 +67,7 @@ import {
   removePidFile,
   getPlatformTimeout,
   cleanupOrphanedProcesses,
+  cleanStalePidFile,
   spawnDaemon,
   createSignalHandler
 } from './infrastructure/ProcessManager.js';
@@ -746,6 +748,9 @@ export class WorkerService {
  * @returns true if worker is healthy (existing or newly started), false on failure
  */
 async function ensureWorkerStarted(port: number): Promise<boolean> {
+  // Clean stale PID file first (cheap: 1 fs read + 1 signal-0 check)
+  cleanStalePidFile();
+
   // Check if worker is already running and healthy
   if (await waitForHealth(port, 1000)) {
     const versionCheck = await checkVersionMatch(port);
@@ -756,7 +761,7 @@ async function ensureWorkerStarted(port: number): Promise<boolean> {
       });
 
       await httpShutdown(port);
-      const freed = await waitForPortFree(port, getPlatformTimeout(15000));
+      const freed = await waitForPortFree(port, getPlatformTimeout(HOOK_TIMEOUTS.PORT_IN_USE_WAIT));
       if (!freed) {
         logger.error('SYSTEM', 'Port did not free up after shutdown for version mismatch restart', { port });
         return false;
@@ -772,7 +777,7 @@ async function ensureWorkerStarted(port: number): Promise<boolean> {
   const portInUse = await isPortInUse(port);
   if (portInUse) {
     logger.info('SYSTEM', 'Port in use, waiting for worker to become healthy');
-    const healthy = await waitForHealth(port, getPlatformTimeout(15000));
+    const healthy = await waitForHealth(port, getPlatformTimeout(HOOK_TIMEOUTS.PORT_IN_USE_WAIT));
     if (healthy) {
       logger.info('SYSTEM', 'Worker is now healthy');
       return true;
@@ -799,7 +804,7 @@ async function ensureWorkerStarted(port: number): Promise<boolean> {
   // PID file is written by the worker itself after listen() succeeds
   // This is race-free and works correctly on Windows where cmd.exe PID is useless
 
-  const healthy = await waitForHealth(port, getPlatformTimeout(30000));
+  const healthy = await waitForHealth(port, getPlatformTimeout(HOOK_TIMEOUTS.POST_SPAWN_WAIT));
   if (!healthy) {
     removePidFile();
     logger.error('SYSTEM', 'Worker failed to start (health check timeout)');
@@ -871,7 +876,7 @@ async function main() {
       // PID file is written by the worker itself after listen() succeeds
       // This is race-free and works correctly on Windows where cmd.exe PID is useless
 
-      const healthy = await waitForHealth(port, getPlatformTimeout(30000));
+      const healthy = await waitForHealth(port, getPlatformTimeout(HOOK_TIMEOUTS.POST_SPAWN_WAIT));
       if (!healthy) {
         removePidFile();
         logger.error('SYSTEM', 'Worker failed to restart');