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/tests/mock-adapter.ts

@@ -1,5 +1,5 @@
 import type { GenerateOptions, StreamChunk } from '@deepseek-ai/dsh-llm'
-import { LlmAdapter } from '@deepseek-ai/dsh-llm'
+import { CallId, LlmAdapter } from '@deepseek-ai/dsh-llm'
 
 /** Helpers to write scripted responses tersely. */
 export function textResponse(text: string): StreamChunk[] {
@@ -12,7 +12,8 @@ export function textResponse(text: string): StreamChunk[] {
   ]
 }
 
-export function toolCallResponse(callId: string, name: string, args: object, text?: string): StreamChunk[] {
+export function toolCallResponse(rawCallId: string, name: string, args: object, text?: string): StreamChunk[] {
+  const callId = CallId(rawCallId)
   const argumentsJson = JSON.stringify(args)
   const chunks: StreamChunk[] = []
   let index = 0