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.

docs/cordis-catalog/services.md

@@ -44,7 +44,7 @@ async resume(ownerCtx: Context, options: ResumeAgentOptions): Promise<AgentHandl
 
 Types: [Agent](../core-data-structures/core.md) · [AgentOptions](../core-data-structures/core.md) · [SessionHeader](../core-data-structures/persistence.md) · [SessionId](../core-data-structures/core.md)
 
-Source: [`packages/core/agent-loop/src/index.ts:407`](../../packages/core/agent-loop/src/index.ts)
+Source: [`packages/core/agent-loop/src/index.ts:398`](../../packages/core/agent-loop/src/index.ts)
 
 ## `ctx.agents` — `AgentRegistry`