feat(protocol): wire session aggregation and API with liveness

Finalizing the backend engine for the Operative Protocol.
- Updated agent-sessions.ts to use the deriveLiveness logic and the 15m protocol threshold.
- Integrated the agentLivenessMap into the session aggregation to provide real-time status in the Hub.
- Updated the GET /api/sessions endpoint to fetch and serve liveness metadata.
- Fixed linting warnings (unused imports) in reservations, sessions, and test files.

This commit completes the backend contract for bb-u6f.6.2, providing the data layer necessary for the upcoming 'War Room' UI enhancements.

OPERATIVE: silver-castle
SESSION: 2026-02-14-1145

tests/lib/agent-protocol.test.ts

@@ -1,8 +1,7 @@
 import test from 'node:test';
 import assert from 'node:assert/strict';
 import { 
-  createProtocolEvent, 
-  type ProtocolEvent 
+  createProtocolEvent
 } from '../../src/lib/agent-protocol';
 
 test('createProtocolEvent generates a valid v1 envelope', () => {