feat(data): complete bb-ui2.15 - Swarm Cards Data Builder

STORY:
The Swarm view needs to show epics as "swarms" - groups of agents
working together on a mission. This requires aggregating bead data
by swarm/epic and computing health statistics.

COLLABORATION:
Created buildSwarmCards function that transforms epic + agent data:

SwarmCard interface:
- id, title, status
- stats: { completed, active, ready, blocked, total }
- agents: AgentRoster[] with liveness
- attention: blocked tasks + stuck agents needing attention
- lastActivity

We also created swarm-molecules.ts for molecular composition patterns
used by the swarm orchestration layer.

DELIVERABLES:
- src/lib/swarm-cards.ts with SwarmCard, AgentRoster types
- src/lib/swarm-molecules.ts for molecular composition
- tests/lib/swarm-cards.test.ts
- tests/lib/swarm-molecules.test.ts

VERIFICATION:
- npm run typecheck: PASS
- npm run lint: PASS
- npm run test: PASS

CLOSES: bb-ui2.15
BLOCKS: bb-ui2.16, bb-ui2.17

tests/lib/swarm-molecules.test.ts

@@ -0,0 +1,88 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { execSync } from 'node:child_process';
+
+import { joinSwarm, leaveSwarm, getSwarmMembers } from '../../src/lib/swarm-molecules';
+import { registerAgent } from '../../src/lib/agent-registry';
+
+const projectRoot = process.cwd();
+const runId = Date.now().toString(36);
+
+// Helper: bd show returns an ARRAY, take first element
+function bdShow(beadId: string): any {
+  const out = execSync(`bd --allow-stale show ${beadId} --json`, { cwd: projectRoot, encoding: 'utf8', timeout: 30000 });
+  const arr = JSON.parse(out);
+  return arr[0] || arr;
+}
+
+test('joinSwarm creates swarm membership', async () => {
+  const agentId = `join-${runId}`;
+  
+  await registerAgent({ name: agentId, role: 'tester' }, { projectRoot });
+  const result = await joinSwarm({ agent: agentId, epicId: 'bb-buff' }, { projectRoot });
+  assert.equal(result.ok, true, `joinSwarm failed: ${result.error?.message}`);
+  
+  const agent = bdShow(`bb-${agentId}`);
+  const hasSwarm = agent.labels?.some((l: string) => l.startsWith('swarm:'));
+  assert.ok(hasSwarm, 'Agent should have swarm label');
+  
+  await leaveSwarm({ agent: agentId }, { projectRoot });
+});
+
+test('joinSwarm switches membership', async () => {
+  const agentId = `switch-${runId}`;
+  
+  await registerAgent({ name: agentId, role: 'tester' }, { projectRoot });
+  await joinSwarm({ agent: agentId, epicId: 'bb-buff' }, { projectRoot });
+  await joinSwarm({ agent: agentId, epicId: 'bb-buff.2' }, { projectRoot });
+
+  const agent = bdShow(`bb-${agentId}`);
+  const swarmLabels = agent.labels?.filter((l: string) => l.startsWith('swarm:')) || [];
+  assert.equal(swarmLabels.length, 1, 'Should have exactly one swarm label');
+  
+  await leaveSwarm({ agent: agentId }, { projectRoot });
+});
+
+test('leaveSwarm removes membership', async () => {
+  const agentId = `leave-${runId}`;
+  
+  await registerAgent({ name: agentId, role: 'tester' }, { projectRoot });
+  await joinSwarm({ agent: agentId, epicId: 'bb-buff' }, { projectRoot });
+  await leaveSwarm({ agent: agentId }, { projectRoot });
+
+  const agent = bdShow(`bb-${agentId}`);
+  const swarmLabels = agent.labels?.filter((l: string) => l.startsWith('swarm:')) || [];
+  assert.equal(swarmLabels.length, 0, 'Should have no swarm labels');
+});
+
+test('getSwarmMembers returns members', async () => {
+  const agent1 = `m1-${runId}`;
+  const agent2 = `m2-${runId}`;
+  
+  await registerAgent({ name: agent1, role: 'tester' }, { projectRoot });
+  await registerAgent({ name: agent2, role: 'tester' }, { projectRoot });
+  await joinSwarm({ agent: agent1, epicId: 'bb-buff' }, { projectRoot });
+  await joinSwarm({ agent: agent2, epicId: 'bb-buff' }, { projectRoot });
+
+  const members = await getSwarmMembers({ swarmId: 'bb-buff' }, { projectRoot });
+  assert.ok(members.includes(agent1), `Should include ${agent1}`);
+  assert.ok(members.includes(agent2), `Should include ${agent2}`);
+  
+  await leaveSwarm({ agent: agent1 }, { projectRoot });
+  await leaveSwarm({ agent: agent2 }, { projectRoot });
+});
+
+test('joinSwarm rejects invalid agent', async () => {
+  const result = await joinSwarm({ agent: 'nonexistent', epicId: 'bb-buff' }, { projectRoot });
+  assert.equal(result.ok, false);
+  assert.equal(result.error?.code, 'AGENT_NOT_FOUND');
+});
+
+test('joinSwarm rejects invalid epic', async () => {
+  const agentId = `invepic-${runId}`;
+  await registerAgent({ name: agentId, role: 'tester' }, { projectRoot });
+  
+  const result = await joinSwarm({ agent: agentId, epicId: 'bb-nonexistent' }, { projectRoot });
+  assert.equal(result.ok, false);
+  assert.equal(result.error?.code, 'EPIC_NOT_FOUND');
+});