fix(tui): surface gateway stderr tail in start_timeout activity (#17112)

* fix(tui): append gateway stderr tail to start_timeout activity

`gateway.start_timeout` previously published only `cwd` + `python`,
which made TUI startup failures hard to disambiguate.  The user saw
`gateway startup timed out · /path/to/python /repo · /logs to inspect`
with no signal whether the actual cause was a wrong python interpreter,
a missing dependency, or a config parse failure.

Plumb a 20-line stderr tail through the event so the most useful lines
land directly in the TUI activity feed, capped to the last 8 non-empty
lines for readability:

* `gatewayClient.ts` — collect `getLogTail(20)` when the readyTimer
  fires and attach it as `payload.stderr_tail`.
* `gatewayTypes.ts`  — extend the `gateway.start_timeout` event union
  with the new optional field.
* `createGatewayEventHandler.ts` — emit the trimmed lines after the
  existing `gateway startup timed out` activity entry, classified
  `error`.

Tests: regression test in `createGatewayEventHandler.test.ts` checks
that `ModuleNotFoundError` / `FileNotFoundError` lines from the tail
land in `getTurnState().activity` so they show up in the UI immediately.

Validation: `npm run type-check` clean, `npm test --run` 390/390.

* review(copilot): filter blanks before slice and cap stderr tail at 120 chars

ui-tui/src/__tests__/createGatewayEventHandler.test.ts

@@ -293,6 +293,27 @@ describe('createGatewayEventHandler', () => {
     expect(appended[1]).toMatchObject({ role: 'assistant', text: 'final answer' })
   })
 
+  it('annotates gateway.start_timeout with stderr tail lines so users can diagnose without /logs', () => {
+    const appended: Msg[] = []
+    const onEvent = createGatewayEventHandler(buildCtx(appended))
+
+    onEvent({
+      payload: {
+        cwd: '/repo',
+        python: '/opt/venv/bin/python',
+        stderr_tail:
+          '[startup] timed out\nModuleNotFoundError: No module named openai\nFileNotFoundError: ~/.hermes/config.yaml'
+      },
+      type: 'gateway.start_timeout'
+    } as any)
+
+    const messages = getTurnState().activity.map(a => a.text)
+
+    expect(messages.some(m => m.includes('gateway startup timed out'))).toBe(true)
+    expect(messages.some(m => m.includes('ModuleNotFoundError'))).toBe(true)
+    expect(messages.some(m => m.includes('FileNotFoundError'))).toBe(true)
+  })
+
   it('anchors inline_diff as its own segment where the edit happened', () => {
     const appended: Msg[] = []
     const onEvent = createGatewayEventHandler(buildCtx(appended))