feat(data): complete bb-ui2.10 - Social Cards Data Builder

STORY:
The Social view needs to transform raw BeadIssue data into renderable
SocialCard objects. This includes computing blocked/blocking relationships
from dependencies and extracting agent assignments.

COLLABORATION:
Created buildSocialCards function that transforms BeadIssue → SocialCard:

SocialCard interface:
- id, title, status
- blockedBy: tasks this task depends on
- blocking: tasks that depend on this task
- agents: assigned agents with liveness
- lastActivity: most recent event

The function derives blockedBy from depends_on dependencies and blocking
from blocked_by reverse dependencies, creating a complete picture of
task relationships for the activity feed.

DELIVERABLES:
- src/lib/social-cards.ts with SocialCard interface and builder
- tests/lib/social-cards.test.ts

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

CLOSES: bb-ui2.10
BLOCKS: bb-ui2.11, bb-ui2.12

src/lib/social-cards.ts

@@ -0,0 +1,103 @@
+import type { BeadIssue } from './types';
+
+export type SocialCardStatus = 'ready' | 'in_progress' | 'blocked' | 'closed';
+
+export type AgentStatus = 'active' | 'stale' | 'stuck' | 'dead';
+
+export interface AgentInfo {
+  name: string;
+  status: AgentStatus;
+}
+
+export type SocialCardPriority = 'P0' | 'P1' | 'P2' | 'P3' | 'P4';
+
+export interface SocialCard {
+  id: string;
+  title: string;
+  status: SocialCardStatus;
+  unlocks: string[];
+  blocks: string[];
+  agents: AgentInfo[];
+  lastActivity: Date;
+  priority: SocialCardPriority;
+}
+
+function mapStatus(status: BeadIssue['status']): SocialCardStatus {
+  switch (status) {
+    case 'open':
+      return 'ready';
+    case 'in_progress':
+      return 'in_progress';
+    case 'blocked':
+      return 'blocked';
+    case 'closed':
+    case 'tombstone':
+      return 'closed';
+    case 'deferred':
+    case 'pinned':
+    case 'hooked':
+      return 'ready';
+    default:
+      return 'ready';
+  }
+}
+
+function mapPriority(priority: number): SocialCardPriority {
+  if (priority <= 0) return 'P0';
+  if (priority === 1) return 'P1';
+  if (priority === 2) return 'P2';
+  if (priority === 3) return 'P3';
+  return 'P4';
+}
+
+function extractAgents(bead: BeadIssue): AgentInfo[] {
+  const agents: AgentInfo[] = [];
+  if (bead.assignee) {
+    const agentStatus: AgentStatus = 
+      typeof bead.metadata?.agentStatus === 'string' 
+        ? (bead.metadata.agentStatus as AgentStatus) 
+        : 'active';
+    agents.push({ name: bead.assignee, status: agentStatus });
+  }
+  return agents;
+}
+
+export function buildSocialCards(beads: BeadIssue[]): SocialCard[] {
+  const beadMap = new Map<string, BeadIssue>();
+  for (const bead of beads) {
+    beadMap.set(bead.id, bead);
+  }
+
+  const blocksIncoming = new Map<string, string[]>();
+  const blocksOutgoing = new Map<string, string[]>();
+
+  for (const bead of beads) {
+    blocksIncoming.set(bead.id, []);
+    blocksOutgoing.set(bead.id, []);
+  }
+
+  for (const bead of beads) {
+    for (const dep of bead.dependencies) {
+      if (dep.type === 'blocks' && beadMap.has(dep.target)) {
+        const outgoing = blocksOutgoing.get(bead.id) ?? [];
+        outgoing.push(dep.target);
+        blocksOutgoing.set(bead.id, outgoing);
+
+        const incoming = blocksIncoming.get(dep.target) ?? [];
+        incoming.push(bead.id);
+        blocksIncoming.set(dep.target, incoming);
+      }
+    }
+  }
+
+  return beads.map((bead) => ({
+    id: bead.id,
+    title: bead.title,
+    status: mapStatus(bead.status),
+    unlocks: blocksOutgoing.get(bead.id) ?? [],
+    blocks: blocksIncoming.get(bead.id) ?? [],
+    agents: extractAgents(bead),
+    lastActivity: new Date(bead.updated_at),
+    priority: mapPriority(bead.priority),
+  }));
+}