Show the server request id on non-2xx responses

[hbrooks]

What

When an API call fails, agent now prints the server's per-request id so a user can quote it and we can map the failure to an exact log line.

Before:

error: POST /v1/agents/runs failed: 500 Internal Server Error

After:

error: POST /v1/agents/runs failed: 500 Internal Server Error (request id: request_IRig...KV1r)

Pairs with the backend change (ellipsis-dev/ellipsis#5421) that stamps every response with an X-Request-ID header and includes request_id in the 500 body.

Changes

• ApiError gains a requestId field and appends (request id: ...) to its message when present. Commands that print err.message (e.g. ping) get this for free.
• errorDetail becomes parseErrorResponse, returning { detail, requestId } from a single read of the response. The id is taken from the X-Request-ID header (set on every response) and falls back to the body's request_id (present on 500s).

Tests

• parseErrorResponse: reads the id from the header, falls back to the body request_id, and still works for non-JSON bodies that carry the header.
• ApiClient.request: a non-2xx surfaces requestId on the thrown ApiError and in its message.
• Full suite green (107 tests); typecheck, build, and the Bun compile smoke check all pass.