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.

packages/llm/llm-deepseek/README.md

@@ -42,7 +42,7 @@ Every request carries the shared attribution header from dsh-llm's `attributionH
 
 ## Errors
 
-Non-2xx responses throw `LlmError` with stable codes: `AUTH` (401/403), `RATE_LIMIT` (429), `CONTEXT_WINDOW_EXCEEDED` (a 400 whose provider code, type, or message identifies context overflow), `INVALID_REQUEST` (other 400s), `SERVER` (5xx), `HTTP_<status>` otherwise. Protocol violations throw `STREAM_CLOSED` (no `[DONE]`) or `MALFORMED_RESPONSE` (bad JSON payload). Unknown wire `finish_reason`s (e.g. `content_filter`, `insufficient_system_resource`) become `finish {kind: 'error', code: <REASON>}` chunks.
+Non-2xx responses throw `LlmError` with stable codes: `AUTH` (401/403), `RATE_LIMIT` (429), `CONTEXT_WINDOW_EXCEEDED` (a 400 whose provider code, type, or message identifies context overflow), `INVALID_REQUEST` (other 400s), `SERVER` (5xx), `HTTP_<status>` otherwise. A transport failure before any response (DNS, refused connection, TLS, proxy) throws `NETWORK` naming the configured endpoint and chaining fetch's `TypeError: fetch failed` as `cause`, so `errorChain` renders the underlying diagnosis; an abort keeps its `DOMException` so the loop classifies it as cancellation. Protocol violations throw `STREAM_CLOSED` (no `[DONE]`) or `MALFORMED_RESPONSE` (bad JSON payload). Unknown wire `finish_reason`s (e.g. `content_filter`, `insufficient_system_resource`) become `finish {kind: 'error', code: <REASON>}` chunks.
 
 ## Testing