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/llm/src/index.ts

@@ -10,6 +10,7 @@ import { Context, Service } from 'cordis'
 import type { ContentBlock, GenerateOptions, GenerateResult, StreamChunk } from './types.ts'
 import { BlockAssembler } from './assembler.ts'
 
+export * from './brand.ts'
 export * from './types.ts'
 export { BlockAssembler } from './assembler.ts'