fix(cli): pin base_agent and id in agent create example

The `create` example set on `CreateAgent` in `gen.lock` carried synthesized `<value>` and `<id>` placeholders for `base_agent` and `id`. Because named example sets are rendered in compact help and take priority over schema property examples, `gemini-api agent create --help` printed `gemini-api agent create --base-agent <value> --id <id>`.

Add a `create` request-body example (`base_agent: antigravity-preview-05-2026`, `id: research-assistant`, matching `agent get` and `agent delete`) on `POST /{api_version}/agents` in `overlays/cli/examples.yaml`, and regenerate the CLI.

PiperOrigin-RevId: 991030646

Author: Google Team Member

README.md

@@ -474,32 +474,32 @@ Commands are grouped the way `gemini-api --help` shows them. Every command accep
 <!-- Start Request Body Input [stdinpiping] -->
 ## Request Body Input
 
-Commands that accept a request body take it three ways, with a clear priority chain. The examples use `gemini-api environments create`; every body-bearing command works the same way and prints its exact request schema with `--schema`.
+Commands that accept a request body take it three ways, with a clear priority chain. The examples use `gemini-api agent create`; every body-bearing command works the same way and prints its exact request schema with `--schema`.
 
 ### `--body` flag
 
 Provide the entire request body as a JSON string:
 
 ```bash
-gemini-api environments create --body '{"network":{"allowlist":[{"domain":"github.com","transform":[{"Authorization":"Bearer your-token"}]},{"domain":"*.googleapis.com"}]}}'
+gemini-api agent create --body '{"base_agent":"antigravity-preview-05-2026","id":"research-assistant"}'
 ```
 
 ### Stdin piping (lowest priority)
 
 Pipe JSON into any command that accepts a request body:
 
 ```bash
-echo '{"network":{"allowlist":[{"domain":"github.com","transform":[{"Authorization":"Bearer your-token"}]},{"domain":"*.googleapis.com"}]}}' | gemini-api environments create
+echo '{"base_agent":"antigravity-preview-05-2026","id":"research-assistant"}' | gemini-api agent create
 ```
 
 This is useful for chaining commands, reading from files, or scripting:
 
 ```bash
 # Read body from a file
-gemini-api environments create < request.json
+gemini-api agent create < request.json
 
 # Pipe from another command
-curl -s https://example.com/request.json | gemini-api environments create
+curl -s https://example.com/request.json | gemini-api agent create
 ```
 
 ### Priority