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-loop/src/agent.ts

@@ -7,7 +7,7 @@
  */
 
 import type { Context } from 'cordis'
-import type { AgentOptions, AgentStatus, SendOptions } from '@deepseek-ai/dsh-agent'
+import type { AgentId, AgentOptions, AgentStatus, SendOptions } from '@deepseek-ai/dsh-agent'
 import type { Agent } from '@deepseek-ai/dsh-agent'
 import type { ContentBlock, MessageSource } from '@deepseek-ai/dsh-llm'
 import type { Session } from '@deepseek-ai/dsh-session'
@@ -33,7 +33,7 @@ export class LoopAgent implements Agent {
 
   constructor(
     private ctx: Context,
-    public readonly id: string,
+    public readonly id: AgentId,
     public readonly options: AgentOptions,
     public readonly session: Session,
   ) {