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/tui/src/index.ts

@@ -35,6 +35,7 @@ import type { Context } from 'cordis'
 import z from 'schemastery'
 import type { Agent, AgentStatus } from '@deepseek-ai/dsh-agent'
 import type {} from '@deepseek-ai/dsh-agent-loop'
+import { errorChain } from '@deepseek-ai/dsh-llm'
 import type { ContentBlock, StreamChunk } from '@deepseek-ai/dsh-llm'
 import { SessionId, type Session, type SessionEvent, type TodoItem } from '@deepseek-ai/dsh-session'
 import type {
@@ -191,15 +192,6 @@ function displayText(text: string): string {
     `\\x${control.charCodeAt(0).toString(16).padStart(2, '0')}`)
 }
 
-/** Render an arbitrary failure without allowing hostile coercion to escape the UI boundary. */
-function renderThrown(value: unknown): string {
-  try {
-    return String(value)
-  } catch {
-    return '<unrenderable thrown value>'
-  }
-}
-
 /**
  * Theme-agnostic palette built from the standard 16-color ANSI set plus SGR
  * attributes, which every terminal remaps to its active color scheme. Body
@@ -1263,7 +1255,9 @@ export function createTuiChat(
   const disposeError = ctx.on('agent/error', (subject, turn, step, error) => {
     if (subject !== agent) return
     liveErrors.add(`${turn}:${step}`)
-    appendNotice(error.message, 'error')
+    // Full cause chain: wrapper messages like `fetch failed` carry the
+    // actionable transport detail on `cause`.
+    appendNotice(errorChain(error), 'error')
   })
   const disposeAgent = ctx.on('agent/disposed', (subject) => {
     if (subject !== agent) return
@@ -1330,7 +1324,7 @@ export function mountTui(ctx: Context, config: Config, runtime: TuiRuntime): voi
     if (settled || failedSessionId !== sessionId) return
     settled = true
     stopWaiting()
-    runtime.terminal.write(displayText(`ui-tui: session "${sessionId}" failed to start: ${renderThrown(error)}\n`))
+    runtime.terminal.write(displayText(`ui-tui: session "${sessionId}" failed to start: ${errorChain(error)}\n`))
     runtime.exit(1)
   }