fix(persistence): force-save cookie state on graceful shutdown

[shaun0927]

Summary

• Add saveAllStorageState() method to SessionManager that iterates all active sessions and saves their storage state via CDP Network.getAllCookies
• Wire it into the enhancedShutdown handler in src/index.ts with a 5-second timeout to prevent hanging shutdown
• Ensures cookies are persisted before process exit on SIGTERM/SIGINT

Problem

The StorageStateManager watchdog timer uses .unref(), meaning it does not prevent Node.js process exit. If the MCP server is killed (SIGTERM/SIGINT), the final periodic save may not have completed, causing cookie/session loss across restarts.

This is the compounding factor from #606 — even when the persistent profile correctly stores cookies, the storage state backup (which serves as a safety net for headless-shell and cross-session restoration) was not guaranteed to be written on exit.

Changes

File	Change
src/session-manager.ts	Added saveAllStorageState() — iterates all sessions with active StorageStateManager, finds a page per session, calls manager.save()
src/index.ts	Call saveAllStorageState() in enhancedShutdown before stopping stateManager, with 5s timeout

Test plan

• npm run build — clean (zero TypeScript errors)
• npm test — 170 suites passed, 3191 tests pass
• Manual: launch with --auto-launch, authenticate to a site, send SIGTERM, verify storage state JSON exists in ~/.openchrome/storage-state/
• Manual: restart after SIGTERM, verify cookies are restored on first target creation

Closes #606 (compounding factor)

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