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/config-catalog.md

@@ -101,7 +101,7 @@ export interface Config {
 
 Depends on: [`AgentOptions`](core-data-structures/core.md) · [`SessionId`](core-data-structures/core.md)
 
-Source: [`packages/core/agent-loop/src/index.ts:369`](../packages/core/agent-loop/src/index.ts)
+Source: [`packages/core/agent-loop/src/index.ts:360`](../packages/core/agent-loop/src/index.ts)
 
 ## `@deepseek-ai/dsh-agent-spine-demo`
 
@@ -822,7 +822,7 @@ export interface Config {
 }
 ```
 
-Source: [`packages/ui/stdio/src/index.ts:33`](../packages/ui/stdio/src/index.ts)
+Source: [`packages/ui/stdio/src/index.ts:34`](../packages/ui/stdio/src/index.ts)
 
 ## `@deepseek-ai/dsh-stdio-demo`
 
@@ -1266,7 +1266,7 @@ export interface TuiConfig {
 }
 ```
 
-Source: [`packages/ui/tui/src/index.ts:100`](../packages/ui/tui/src/index.ts)
+Source: [`packages/ui/tui/src/index.ts:101`](../packages/ui/tui/src/index.ts)
 
 ## `@deepseek-ai/dsh-user-approval`
 

docs/cordis-catalog/events.md

@@ -366,7 +366,7 @@ A declarative agent entry failed before it could publish a live agent. Consumers
 
 Types: [SessionId](../core-data-structures/core.md)
 
-Source: [`packages/core/agent-loop/src/index.ts:362`](../../packages/core/agent-loop/src/index.ts)
+Source: [`packages/core/agent-loop/src/index.ts:353`](../../packages/core/agent-loop/src/index.ts)
 
 ## `approval/*`
 

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`
 

docs/event-producer-consumer.md

@@ -7,7 +7,7 @@ This matrix shows which packages dispatch each harness-owned event and which pac
 
 | Event | Mode | Declared in | Dispatchers | Listeners |
 | --- | --- | --- | --- | --- |
-| `agent-loop/config-start-failed` | `emit` | [`packages/core/agent-loop/src/index.ts:362`](../packages/core/agent-loop/src/index.ts) | [`agent-loop`](../packages/core/agent-loop) (`events.dispatch`) | [`stdio`](../packages/ui/stdio), [`tui`](../packages/ui/tui) |
+| `agent-loop/config-start-failed` | `emit` | [`packages/core/agent-loop/src/index.ts:353`](../packages/core/agent-loop/src/index.ts) | [`agent-loop`](../packages/core/agent-loop) (`events.dispatch`) | [`stdio`](../packages/ui/stdio), [`tui`](../packages/ui/tui) |
 | `agent/created` | `emit` | [`packages/core/agent/src/types.ts:147`](../packages/core/agent/src/types.ts) | [`agent`](../packages/core/agent) (`events.dispatch`) | [`stdio`](../packages/ui/stdio), [`tui`](../packages/ui/tui) |
 | `agent/disposed` | `emit` | [`packages/core/agent/src/types.ts:156`](../packages/core/agent/src/types.ts) | [`agent`](../packages/core/agent) (`events.dispatch`) | [`agent-loop`](../packages/core/agent-loop), [`stdio`](../packages/ui/stdio), [`tui`](../packages/ui/tui) |
 | `agent/error` | `emit` | [`packages/core/agent/src/types.ts:311`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`emit`) | [`tui`](../packages/ui/tui) |