* 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
a830f25f71