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/tests/tui.spec.ts

@@ -864,7 +864,7 @@ describe('terminal mounting', () => {
     expect(terminal.output).toBe('')
     expect(exit).not.toHaveBeenCalled()
     ctx.emit('agent-loop/config-start-failed', SessionId('main-session'), new Error('resume \u001b]2;failure-controlled\u0007'))
-    expect(terminal.output).toBe('ui-tui: session "main-session" failed to start: Error: resume \\x1b]2;failure-controlled\\x07\n')
+    expect(terminal.output).toBe('ui-tui: session "main-session" failed to start: resume \\x1b]2;failure-controlled\\x07\n')
     expect(exit).toHaveBeenCalledWith(1)
 
     const session = ctx.sessions.create(SessionId('main-session'))
@@ -892,7 +892,7 @@ describe('terminal mounting', () => {
     })
 
     expect(terminal.started).toBe(0)
-    expect(terminal.output).toBe('ui-tui: session "main-session" failed to start: <unrenderable thrown value>\n')
+    expect(terminal.output).toBe('ui-tui: session "main-session" failed to start: <unrenderable value>\n')
     expect(exit).toHaveBeenCalledWith(1)
     await ctx.fiber.dispose()
   })