Add branded ID types: CallId, SessionId, AgentId

Nominal string types via a unique-symbol brand (zero runtime cost):
an AgentId can no longer be passed where a CallId is expected. Each
core package brands the IDs it owns — CallId in dsh-llm (tool-call
correlation across blocks, chunks, session events, and execution
results), SessionId in dsh-session, AgentId in dsh-agent. Construction
goes through same-named factory functions; public string-in APIs
(sessions.create, agentLoop.create) keep accepting plain strings and
brand internally. Policy note in the brand module: brand IDs that
cross package boundaries, not every string.

packages/agent/tests/agent.spec.ts

@@ -1,13 +1,14 @@
 import { describe, expect, it } from 'vitest'
 import { Context } from 'cordis'
-import { Session } from '@deepseek-ai/dsh-session'
-import AgentRegistry, { Agent } from '@deepseek-ai/dsh-agent'
+import { Session, SessionId } from '@deepseek-ai/dsh-session'
+import AgentRegistry, { Agent, AgentId } from '@deepseek-ai/dsh-agent'
 
-function stubAgent(id: string): Agent {
+function stubAgent(rawId: string): Agent {
+  const id = AgentId(rawId)
   return {
     id,
     options: {},
-    session: new Session(`${id}-session`),
+    session: new Session(SessionId(`${id}-session`)),
     status: 'idle',
     send() {},
     steer() {},