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/index.ts

@@ -20,7 +20,7 @@ import type {
   ResumeAgentOptions,
   SessionStartSource,
 } from '@deepseek-ai/dsh-agent'
-import type {} from '@deepseek-ai/dsh-llm'
+import { errorChain } from '@deepseek-ai/dsh-llm'
 import { SessionId } from '@deepseek-ai/dsh-session'
 import type { Session, SessionHeader } from '@deepseek-ai/dsh-session'
 import type {} from '@deepseek-ai/dsh-system-prompt'
@@ -41,15 +41,6 @@ const INACTIVE_STATES: ReadonlySet<FiberState> = new Set([
   FiberState.FAILED,
 ])
 
-/** Render an arbitrary thrown value without letting coercion escape containment. */
-function renderThrown(value: unknown): string {
-  try {
-    return String(value)
-  } catch {
-    return '<unrenderable thrown value>'
-  }
-}
-
 /** Factory-level ownership of every preparing or live transaction. */
 class FactoryOwnership {
   private accepting = true
@@ -475,16 +466,16 @@ export class AgentLoop extends Service implements AgentFactory {
     error: unknown,
   ): void {
     if (!this.ownership.isActive()) return
-    this.ctx.logger.warn(`agent "${configId}": config-driven ${action} of "${sessionId}" failed: ${renderThrown(error)}`)
+    this.ctx.logger.warn(`agent "${configId}": config-driven ${action} of "${sessionId}" failed: ${errorChain(error)}`)
     const args: unknown[] = ['agent-loop/config-start-failed', sessionId, error]
     for (const callback of this.ctx.events.dispatch('emit', args)) {
       try {
         const returned: unknown = callback(...args)
         void Promise.resolve(returned).catch((listenerError: unknown) => {
-          this.ctx.logger.warn(`agent "${configId}": config-start-failed listener rejected: ${renderThrown(listenerError)}`)
+          this.ctx.logger.warn(`agent "${configId}": config-start-failed listener rejected: ${errorChain(listenerError)}`)
         })
       } catch (listenerError: unknown) {
-        this.ctx.logger.warn(`agent "${configId}": config-start-failed listener threw: ${renderThrown(listenerError)}`)
+        this.ctx.logger.warn(`agent "${configId}": config-start-failed listener threw: ${errorChain(listenerError)}`)
       }
     }
   }