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/src/types.ts

@@ -9,7 +9,15 @@
  * @module @deepseek-ai/dsh-agent/types
  */
 
-import type { ContentBlock, GenerateOptions, Message, MessageSource, StreamChunk } from '@deepseek-ai/dsh-llm'
+import type { Branded, ContentBlock, GenerateOptions, Message, MessageSource, StreamChunk } from '@deepseek-ai/dsh-llm'
+
+/** Identifies one live agent in the registry. */
+export type AgentId = Branded<'AgentId'>
+
+/** Brand a string as an {@link AgentId}. */
+export function AgentId(id: string): AgentId {
+  return id as AgentId
+}
 import type { Session, TurnEndReason } from '@deepseek-ai/dsh-session'
 
 /**
@@ -36,7 +44,7 @@ export type AgentStatus = 'idle' | 'running' | 'disposed'
  * package should depend on the implementation.
  */
 export interface Agent {
-  readonly id: string
+  readonly id: AgentId
   readonly options: AgentOptions
   readonly session: Session
   readonly status: AgentStatus