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/core/agent-loop/tests/config-session-id.spec.ts

@@ -217,14 +217,14 @@ describe('config-driven session id', () => {
     })
 
     await expect.poll(() => warn).toHaveBeenCalledWith(expect.stringContaining(
-      'config-driven restore of "stdio-exact-failure" failed: Error: persistence index failed',
+      'config-driven restore of "stdio-exact-failure" failed: persistence index failed',
     ))
     expect(failures).toEqual([{ sessionId: SessionId('stdio-exact-failure'), error: failure }])
     expect(warn).toHaveBeenCalledWith(
-      'agent "main": config-start-failed listener threw: Error: failure observer failed',
+      'agent "main": config-start-failed listener threw: failure observer failed',
     )
     await expect.poll(() => warn).toHaveBeenCalledWith(
-      'agent "main": config-start-failed listener rejected: Error: async failure observer failed',
+      'agent "main": config-start-failed listener rejected: async failure observer failed',
     )
     expect(ctx.agents.get(SessionId('stdio-exact-failure'))).toBeUndefined()
     warn.mockRestore()
@@ -256,13 +256,13 @@ describe('config-driven session id', () => {
 
     await expect.poll(() => failures).toEqual([unrenderable])
     expect(warn).toHaveBeenCalledWith(
-      'agent "main": config-driven restore of "stdio-exact-unrenderable" failed: <unrenderable thrown value>',
+      'agent "main": config-driven restore of "stdio-exact-unrenderable" failed: <unrenderable value>',
     )
     expect(warn).toHaveBeenCalledWith(
-      'agent "main": config-start-failed listener threw: <unrenderable thrown value>',
+      'agent "main": config-start-failed listener threw: <unrenderable value>',
     )
     await expect.poll(() => warn).toHaveBeenCalledWith(
-      'agent "main": config-start-failed listener rejected: <unrenderable thrown value>',
+      'agent "main": config-start-failed listener rejected: <unrenderable value>',
     )
     await ctx.fiber.dispose()
   })