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/ui/stdio/README.md

@@ -1,6 +1,6 @@
 # @deepseek-ai/dsh-stdio
 
-The terminal readline front door for DeepSeek Harness agents. It reads prompts from stdin, sends or steers them through `ctx.agents`, renders the durable `session/event` transcript to stdout, and answers `ctx.userInteraction` requests in the same terminal.
+The terminal readline front door for DeepSeek Harness agents. It reads prompts from stdin, sends or steers them through `ctx.agents`, renders the durable `session/event` transcript to stdout, and answers `ctx.userInteraction` requests in the same terminal. Failed turns render their durable `turn/end` reason — `[turn failed <code>]`, `[turn aborted]`, `[turn rejected]`, `[turn interrupted …]`, or the output-token-limit notice — so a provider or network failure is never silent; unknown merge-extended reason kinds fall through as ordinary turn ends.
 
 This package owns the terminal channel only. It injects `agents` and `userInteraction`, then drives an agent created or resumed by app or developer code. The agent spine, agent lifecycle, console logger, and model-facing [`ask_user_question`](../tool-ask-user/README.md) tool remain separate composition entries.