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,
   ) {

packages/agent-loop/src/index.ts

@@ -9,6 +9,7 @@
 
 import { Context, Service } from 'cordis'
 import z from 'schemastery'
+import { AgentId } from '@deepseek-ai/dsh-agent'
 import type { AgentOptions } from '@deepseek-ai/dsh-agent'
 import type {} from '@deepseek-ai/dsh-llm'
 import type {} from '@deepseek-ai/dsh-session'
@@ -67,7 +68,7 @@ export class AgentLoop extends Service {
    */
   create(id: string, options: AgentOptions = {}): LoopAgent {
     const session = this.ctx.sessions.create(`${id}-session`)
-    const agent = new LoopAgent(this.ctx, id, options, session)
+    const agent = new LoopAgent(this.ctx, AgentId(id), options, session)
     // Generator effect: stop and unregister are independent disposables
     // (LIFO), so a throwing stop() cannot leak the registry entry.
     this.ctx.effect(function* (this: AgentLoop) {