feat(logic): establish derived-activity engine and agent-session protocols

Today we reached a major architectural conclusion: project history shouldn't be stored, it should be derived. We rejected the overhead of a separate SQLite event store in favor of an O(N) snapshot-diffing engine that computes human-readable narratives directly from the issues.jsonl source of truth.

Key Triumphs:
- Implemented O(N) diffing algorithm in src/lib/snapshot-differ.ts that transforms raw JSONL into 16 distinct social event types.
- Engineered a file-based persistence layer (src/lib/activity-persistence.ts) to solve the 'Next.js HMR Wiped My Memory' bug, ensuring project heartbeat survives server restarts.
- Developed the agent-session data model that unifies Beads, Activity, and Cross-Agent Mail into a single 'Mission' context.

Raw Honest Moment:
We struggled for over an hour with 'missing history' before realizing that development-mode reloads were purging our in-memory buffers. The shift to a file-backed ring buffer was a reactive pivot that became a core project strength.

tests/lib/activity.test.ts

@@ -0,0 +1,57 @@
+import { describe, it } from 'node:test';
+import assert from 'node:assert';
+import type { ActivityEvent, ActivityEventKind } from '../../src/lib/activity';
+
+describe('Activity Event Model (bb-xhm.1)', () => {
+  it('should support all 16 required transition types in ActivityEventKind', () => {
+    const kinds: ActivityEventKind[] = [
+      'created',
+      'closed',
+      'reopened',
+      'status_changed',
+      'priority_changed',
+      'assignee_changed',
+      'type_changed',
+      'title_changed',
+      'description_changed',
+      'labels_changed',
+      'dependency_added',
+      'dependency_removed',
+      'comment_added',
+      'due_date_changed',
+      'estimate_changed',
+      'field_changed',
+    ];
+
+    assert.strictEqual(kinds.length, 16, 'Should have exactly 16 transition types');
+    
+    // Verify specific important types are present
+    assert.ok(kinds.includes('created'));
+    assert.ok(kinds.includes('closed'));
+    assert.ok(kinds.includes('reopened'));
+    assert.ok(kinds.includes('comment_added'));
+  });
+
+  it('should allow creating a valid ActivityEvent object', () => {
+    const event: ActivityEvent = {
+      id: 'evt-123',
+      kind: 'status_changed',
+      beadId: 'bb-1',
+      beadTitle: 'Test Bead',
+      projectId: 'proj-1',
+      projectName: 'Test Project',
+      timestamp: new Date().toISOString(),
+      actor: 'zenchantlive',
+      payload: {
+        field: 'status',
+        from: 'open',
+        to: 'in_progress',
+      },
+    };
+
+    assert.strictEqual(event.kind, 'status_changed');
+    assert.strictEqual(event.payload.field, 'status');
+    assert.strictEqual(event.payload.from, 'open');
+    assert.strictEqual(event.payload.to, 'in_progress');
+  });
+});