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`
 
@@ -858,7 +858,7 @@ 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`
 
@@ -1302,7 +1302,7 @@ 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`