chore: add utility scripts and additional test coverage

STORY:
Development requires supporting scripts and comprehensive test coverage.
This commit adds utility scripts and extends test suites.

COLLABORATION:
- scripts/capture-sessions.mjs: Screenshot capture for sessions hub
- scripts/capture-timeline.mjs: Timeline view capture utility
- sessions-header-logic.ts: Session header business logic
- Additional test files for sessions, hooks, and snapshot diffing

tests/components/sessions/sessions-header.test.ts

@@ -0,0 +1,85 @@
+import { describe, it } from 'node:test';
+import assert from 'node:assert';
+import type { AgentRecord } from '../../../src/lib/agent-registry';
+
+interface SwarmGroup {
+  swarmId: string;
+  swarmLabel: string;
+  members: AgentRecord[];
+}
+
+function groupAgentsBySwarm(
+  agents: AgentRecord[],
+  swarmGroups: SwarmGroup[]
+): { swarmGroups: SwarmGroup[]; unassigned: AgentRecord[] } {
+  const nonEmptySwarmGroups = swarmGroups.filter(g => g.members.length > 0);
+  const assignedIds = new Set(nonEmptySwarmGroups.flatMap(g => g.members.map(m => m.agent_id)));
+  const unassigned = agents.filter(a => !assignedIds.has(a.agent_id));
+  return { swarmGroups: nonEmptySwarmGroups, unassigned };
+}
+
+describe('SessionsHeader: Agent Grouping', () => {
+  const mockAgent = (id: string): AgentRecord => ({
+    agent_id: id,
+    display_name: id,
+    role: 'agent',
+    status: 'idle',
+    created_at: new Date().toISOString(),
+    last_seen_at: new Date().toISOString(),
+    version: 1,
+  });
+
+  it('groups agents by swarm', () => {
+    const agents = [
+      mockAgent('agent-1'),
+      mockAgent('agent-2'),
+      mockAgent('agent-3'),
+    ];
+
+    const swarmGroups: SwarmGroup[] = [
+      {
+        swarmId: 'bb-buff',
+        swarmLabel: 'bb-buff',
+        members: [agents[0], agents[1]],
+      },
+    ];
+
+    const result = groupAgentsBySwarm(agents, swarmGroups);
+
+    assert.equal(result.swarmGroups.length, 1);
+    assert.equal(result.swarmGroups[0].members.length, 2);
+    assert.equal(result.unassigned.length, 1);
+    assert.equal(result.unassigned[0].agent_id, 'agent-3');
+  });
+
+  it('shows fallback bucket for unassigned agents', () => {
+    const agents = [
+      mockAgent('agent-1'),
+      mockAgent('agent-2'),
+    ];
+
+    const swarmGroups: SwarmGroup[] = [];
+
+    const result = groupAgentsBySwarm(agents, swarmGroups);
+
+    assert.equal(result.swarmGroups.length, 0);
+    assert.equal(result.unassigned.length, 2);
+  });
+
+  it('handles empty swarm groups', () => {
+    const agents = [mockAgent('agent-1')];
+
+    const swarmGroups: SwarmGroup[] = [
+      {
+        swarmId: 'bb-empty',
+        swarmLabel: 'bb-empty',
+        members: [],
+      },
+    ];
+
+    const result = groupAgentsBySwarm(agents, swarmGroups);
+
+    assert.equal(result.swarmGroups.length, 0, 'Empty swarm groups should not render');
+    assert.equal(result.unassigned.length, 1);
+  });
+});