Render error cause chains at every diagnostic seam

A TUI run against an unreachable endpoint failed with only 'fetch failed':
undici wraps transport failures in a bare TypeError whose diagnosis lives
on .cause, and every diagnostic seam rendered only error.message. The
readline front door additionally rendered failed turns as pure silence.

- dsh-llm: new errorChain(value) renders the full cause chain and
  AggregateError members with circular/hostile-coercion containment.
- llm-deepseek: pre-response transport failures throw LlmError('NETWORK')
  naming the endpoint and chaining the fetch TypeError; aborts keep their
  DOMException so the loop still classifies them as cancellation.
- agent-loop: durable turn/end error messages and logger warnings render
  through errorChain; local renderThrown copies removed.
- ui-stdio: failure turn/end reasons now render ([turn failed <code>],
  [turn aborted], [turn rejected], output-token-limit); startup-failure
  logs use errorChain.
- ui-tui: agent/error notices and the startup-failure line use errorChain.

packages/core/agent-loop/src/agent.ts

@@ -10,7 +10,7 @@ import type { Context } from 'cordis'
 import { agentEvents } from '@deepseek-ai/dsh-agent'
 import type { AgentOptions, AgentStatus, HookContext, InjectOptions, SendOptions } from '@deepseek-ai/dsh-agent'
 import type { Agent } from '@deepseek-ai/dsh-agent'
-import { deepFreeze } from '@deepseek-ai/dsh-llm'
+import { deepFreeze, errorChain } from '@deepseek-ai/dsh-llm'
 import type { ContentBlock, MessageSource } from '@deepseek-ai/dsh-llm'
 import { snapshotJsonValue, type Session, type SessionId } from '@deepseek-ai/dsh-session'
 import { Inbox, type InboxMessage } from './inbox.ts'
@@ -291,7 +291,7 @@ export class ReactLoopAgent implements Agent {
       if (turnRecorded) {
         // Through the store's flush (the carrier owner), never a raw parallel.
         const flush = this.loopCtx.sessions.flush(this.session).catch((error: unknown) => {
-          const rendered = renderThrown(error)
+          const rendered = errorChain(error)
           const err = error instanceof Error ? error : new Error(rendered)
           this.loopCtx.logger.warn(`agent "${this.id}": flush after idle injection failed: ${rendered}`)
           agentEvents(this.loopCtx, this).emit('agent/error', turn, 0, err)
@@ -446,7 +446,3 @@ export class ReactLoopAgent implements Agent {
   }
 }
 
-/** Render an ordinary thrown value for the error event and log. */
-function renderThrown(value: unknown): string {
-  return value instanceof Error ? value.message : String(value)
-}