feat(protocol): implement core backend engine for Operative Protocol

Our collaboration led to a rigorous 'Session Constitution' where we prioritized observability and concurrency safety.
I've delivered the first four pillars of the backend engine:
1. Liveness Registry: Heartbeat logic and derivation of active/stale/evicted states based on the 15m threshold.
2. Overlap Classifier: Canonical path normalization (Windows-aware) and exact/partial overlap detection.
3. Takeover Rules: Enforced discipline where active agents are protected, while stale/evicted ones can be overtaken via --takeover-stale.
4. Protocol Schema: Establishing the v1 envelope for high-fidelity agent signaling.

TDD was applied throughout, with 100% pass rate on the new liveness, overlap, takeover, and protocol tests.

tests/lib/agent-protocol.test.ts

@@ -0,0 +1,29 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { 
+  createProtocolEvent, 
+  type ProtocolEvent 
+} from '../../src/lib/agent-protocol';
+
+test('createProtocolEvent generates a valid v1 envelope', () => {
+  const event = createProtocolEvent({
+    event_type: 'HANDOFF',
+    project_root: '/work/project',
+    bead_id: 'bb-123',
+    from_agent: 'agent-a',
+    to_agent: 'agent-b',
+    scope: 'src/lib/*',
+    payload: {
+      subject: 'Ready for review',
+      summary: 'Implemented feature X',
+      next_action: 'Please run tests',
+      requires_ack: true
+    }
+  }, { now: () => '2026-02-14T10:00:00.000Z', idGenerator: () => 'proto_1' });
+
+  assert.equal(event.version, 'v1');
+  assert.equal(event.event_type, 'HANDOFF');
+  assert.equal(event.id, 'proto_1');
+  assert.equal(event.created_at, '2026-02-14T10:00:00.000Z');
+  assert.equal(event.payload.subject, 'Ready for review');
+});