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.

website/zh-CN/api/harness/agent-loop.md

@@ -6,7 +6,7 @@
 
 Concrete agent factory and driver service.
 
-[Source](https://github.com/deepseek-harness/deepseek-harness/blob/master/packages/core/agent-loop/src/index.ts#L407)
+[Source](https://github.com/deepseek-harness/deepseek-harness/blob/master/packages/core/agent-loop/src/index.ts#L398)
 
 ### ctx.agentLoop.create(id, options?, meta?)
 
@@ -31,7 +31,7 @@ Create an agent and session under one caller-supplied identity, owned by the acc
 
 **Returns** the published running agent.
 
-[Source](https://github.com/deepseek-harness/deepseek-harness/blob/master/packages/core/agent-loop/src/index.ts#L542)
+[Source](https://github.com/deepseek-harness/deepseek-harness/blob/master/packages/core/agent-loop/src/index.ts#L533)
 
 ### ctx.agentLoop.createAgent(ownerCtx, options)
 
@@ -52,7 +52,7 @@ Create an owned agent on a caller-supplied session id.
 
 **Returns** the published handle.
 
-[Source](https://github.com/deepseek-harness/deepseek-harness/blob/master/packages/core/agent-loop/src/index.ts#L564)
+[Source](https://github.com/deepseek-harness/deepseek-harness/blob/master/packages/core/agent-loop/src/index.ts#L555)
 
 ### ctx.agentLoop.resume(ownerCtx, options)
 
@@ -73,4 +73,4 @@ Resume an owned agent from the configured persistence service.
 
 **Returns** the published handle.
 
-[Source](https://github.com/deepseek-harness/deepseek-harness/blob/master/packages/core/agent-loop/src/index.ts#L596)
+[Source](https://github.com/deepseek-harness/deepseek-harness/blob/master/packages/core/agent-loop/src/index.ts#L587)

website/zh-CN/api/harness/events.md

@@ -423,7 +423,7 @@ A declarative agent entry failed before it could publish a live agent. Consumers
 - `sessionId` — exact shared agent/session identity that failed startup.
 - `error` — persistence, setup, or publication failure.
 
-[Source](https://github.com/deepseek-harness/deepseek-harness/blob/master/packages/core/agent-loop/src/index.ts#L362)
+[Source](https://github.com/deepseek-harness/deepseek-harness/blob/master/packages/core/agent-loop/src/index.ts#L353)
 
 ## approval/*