feat: add elastic api for untyped HTTP

The command surface is generated from specs, but agents still hit gaps: preview Kibana routes, custom headers, undocumented query params, and hitting an arbitrary path during incidents.

Add `elastic api ` that uses the active context's ES, Kibana, or Cloud base URL (`--service es|kb|cloud`). Reuse existing auth, `--json`, `--dry-run`, `--input-file`. No pagination helper in MVP.

Prefer typed commands. Use `elastic api` only when no generated command exists.

Acceptance: `elastic api GET / --service es --dry-run` prints the resolved request. A real GET `/` against a configured ES returns cluster info. Path params are encoded. `redirect: 'error'` on the fetch.