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/src/index.ts

@@ -15,6 +15,7 @@ import type { Readable, Writable } from 'node:stream'
 import type { Context } from 'cordis'
 import z from 'schemastery'
 import type { Agent } from '@deepseek-ai/dsh-agent'
+import { errorChain } from '@deepseek-ai/dsh-llm'
 import type {} from '@deepseek-ai/dsh-agent-loop'
 import { SessionId } from '@deepseek-ai/dsh-session'
 import {
@@ -62,15 +63,6 @@ function isTTYPair(input: Readable, output: Writable): boolean {
   return Boolean((input as { isTTY?: boolean }).isTTY && (output as { isTTY?: boolean }).isTTY)
 }
 
-/** 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>'
-  }
-}
-
 interface PendingQuestion {
   request: AskUserQuestionRequest
   questionIndex: number
@@ -138,6 +130,21 @@ export function createStdioChat(ctx: Context, config: Config, runtime: StdioRunt
     } else if (event.type === 'turn/end') {
       if (inReasoning) output.write('\x1B[0m')
       inReasoning = false
+      // Failure reasons must reach the terminal: turn/end is the durable record
+      // of an in-turn failure, and without this line a failed turn renders as
+      // silence. Merge-extensible unknown kinds fall through as ordinary ends.
+      const { reason } = event.data
+      if (reason.kind === 'error') {
+        output.write(`\n[turn failed${reason.code === undefined ? '' : ` ${reason.code}`}] ${reason.message}`)
+      } else if (reason.kind === 'aborted') {
+        output.write(`\n[turn aborted]${reason.reason === undefined ? '' : ` ${reason.reason}`}`)
+      } else if (reason.kind === 'rejected') {
+        output.write(`\n[turn rejected] ${reason.reason}`)
+      } else if (reason.kind === 'max-tokens') {
+        output.write('\n[turn hit the output-token limit]')
+      } else if (reason.kind === 'interrupted') {
+        output.write('\n[turn interrupted by a previous process exit]')
+      }
       output.write('\n> ')
     } else if (event.type === 'tool/call') {
       const { name: toolName, arguments: args } = event.data
@@ -235,7 +242,7 @@ export function createStdioChat(ctx: Context, config: Config, runtime: StdioRunt
       queuedInput.length = 0
       submittedWork = sawRunning
       if (dropped > 0) {
-        ctx.logger.error(`ui-stdio: main agent failed to start; dropped queued stdin (${dropped} line(s)): ${renderThrown(error)}`)
+        ctx.logger.error(`ui-stdio: main agent failed to start; dropped queued stdin (${dropped} line(s)): ${errorChain(error)}`)
       }
       maybeExit()
     })
@@ -395,7 +402,7 @@ export function createStdioChat(ctx: Context, config: Config, runtime: StdioRunt
       const text = line.trim()
       if (!text) return
       if (failedStartup !== undefined) {
-        ctx.logger.error(`ui-stdio: main agent failed to start; dropped queued stdin (1 line(s)): ${renderThrown(failedStartup.error)}`)
+        ctx.logger.error(`ui-stdio: main agent failed to start; dropped queued stdin (1 line(s)): ${errorChain(failedStartup.error)}`)
         return
       }
       const agent = target