refactor(types): add GroupId and AgentId branded types to ids.ts

Extend the branded type system with two new domain types for
btmsg/bttask agent and group identifiers. Apply to groups.ts
interfaces including agentToProject() domain crossing cast.

v2/src/lib/types/groups.ts

@@ -1,8 +1,9 @@
 import type { ProviderId } from '../providers/types';
 import type { AnchorBudgetScale } from './anchors';
+import type { ProjectId, GroupId, AgentId } from './ids';
 
 export interface ProjectConfig {
-  id: string;
+  id: ProjectId;
   name: string;
   identifier: string;
   description: string;
@@ -35,8 +36,9 @@ export const AGENT_ROLE_ICONS: Record<string, string> = {
 
 /** Convert a GroupAgentConfig to a ProjectConfig for unified rendering */
 export function agentToProject(agent: GroupAgentConfig, groupCwd: string): ProjectConfig {
+  // Agent IDs serve as project IDs in the workspace (agents render as project boxes)
   return {
-    id: agent.id,
+    id: agent.id as unknown as ProjectId,
     name: agent.name,
     identifier: agent.role,
     description: `${agent.role.charAt(0).toUpperCase() + agent.role.slice(1)} agent`,
@@ -58,7 +60,7 @@ export type GroupAgentStatus = 'active' | 'sleeping' | 'stopped';
 
 /** Group-level agent configuration */
 export interface GroupAgentConfig {
-  id: string;
+  id: AgentId;
   name: string;
   role: GroupAgentRole;
   model?: string;
@@ -70,7 +72,7 @@ export interface GroupAgentConfig {
 }
 
 export interface GroupConfig {
-  id: string;
+  id: GroupId;
   name: string;
   projects: ProjectConfig[];
   /** Group-level orchestration agents (Tier 1) */
@@ -80,7 +82,7 @@ export interface GroupConfig {
 export interface GroupsFile {
   version: number;
   groups: GroupConfig[];
-  activeGroupId: string;
+  activeGroupId: GroupId;
 }
 
 /** Derive a project identifier from a name: lowercase, spaces to dashes */

v2/src/lib/types/ids.test.ts

@@ -1,5 +1,11 @@
 import { describe, it, expect } from 'vitest';
-import { SessionId, ProjectId, type SessionId as SessionIdType, type ProjectId as ProjectIdType } from './ids';
+import {
+  SessionId, ProjectId, GroupId, AgentId,
+  type SessionId as SessionIdType,
+  type ProjectId as ProjectIdType,
+  type GroupId as GroupIdType,
+  type AgentId as AgentIdType,
+} from './ids';
 
 describe('branded types', () => {
   describe('SessionId', () => {
@@ -41,12 +47,50 @@ describe('branded types', () => {
     });
   });
 
+  describe('GroupId', () => {
+    it('creates a GroupId from a string', () => {
+      const id = GroupId('grp-abc');
+      expect(id).toBe('grp-abc');
+    });
+
+    it('is usable as a Map key', () => {
+      const map = new Map<GroupIdType, string>();
+      const id = GroupId('grp-1');
+      map.set(id, 'test-group');
+      expect(map.get(id)).toBe('test-group');
+    });
+  });
+
+  describe('AgentId', () => {
+    it('creates an AgentId from a string', () => {
+      const id = AgentId('agent-manager');
+      expect(id).toBe('agent-manager');
+    });
+
+    it('is usable as a Map key', () => {
+      const map = new Map<AgentIdType, number>();
+      const id = AgentId('a1');
+      map.set(id, 99);
+      expect(map.get(id)).toBe(99);
+    });
+
+    it('equality works between two AgentIds with same value', () => {
+      const a = AgentId('a1');
+      const b = AgentId('a1');
+      expect(a === b).toBe(true);
+    });
+  });
+
   describe('type safety (compile-time)', () => {
-    it('both types are strings at runtime', () => {
+    it('all four types are strings at runtime', () => {
       const sid = SessionId('s1');
       const pid = ProjectId('p1');
+      const gid = GroupId('g1');
+      const aid = AgentId('a1');
       expect(typeof sid).toBe('string');
       expect(typeof pid).toBe('string');
+      expect(typeof gid).toBe('string');
+      expect(typeof aid).toBe('string');
     });
   });
 });

v2/src/lib/types/ids.ts

@@ -1,4 +1,4 @@
-// Branded types for domain identifiers — prevents accidental swapping of sessionId/projectId
+// Branded types for domain identifiers — prevents accidental swapping of IDs across domains.
 // These are compile-time only; at runtime they are plain strings.
 
 /** Unique identifier for an agent session */
@@ -7,6 +7,12 @@ export type SessionId = string & { readonly __brand: 'SessionId' };
 /** Unique identifier for a project */
 export type ProjectId = string & { readonly __brand: 'ProjectId' };
 
+/** Unique identifier for a project group */
+export type GroupId = string & { readonly __brand: 'GroupId' };
+
+/** Unique identifier for an agent in the btmsg/bttask system */
+export type AgentId = string & { readonly __brand: 'AgentId' };
+
 /** Create a SessionId from a raw string */
 export function SessionId(value: string): SessionId {
   return value as SessionId;
@@ -16,3 +22,13 @@ export function SessionId(value: string): SessionId {
 export function ProjectId(value: string): ProjectId {
   return value as ProjectId;
 }
+
+/** Create a GroupId from a raw string */
+export function GroupId(value: string): GroupId {
+  return value as GroupId;
+}
+
+/** Create an AgentId from a raw string */
+export function AgentId(value: string): AgentId {
+  return value as AgentId;
+}