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.
