Add cloud executor registration to exec-server (#19575)

## Summary
This PR adds the first `codex-rs` milestone for remote-exec e2e: a local
`codex exec-server` can now register itself with
`codex-cloud-environments` and attach to the returned rendezvous
websocket.

At a high level, `codex exec-server --cloud ...` now:
- loads ChatGPT auth from normal Codex config
- registers an executor with `codex-cloud-environments`
- receives a signed rendezvous websocket URL
- serves the existing exec-server JSON-RPC protocol over that websocket

## What Changed
- Added `--cloud`, `--cloud-base-url`, `--cloud-environment-id`, and
`--cloud-name` to `codex exec-server`
- Added a new `exec-server/src/cloud.rs` module that handles:
  - registration requests
  - auth/header setup
  - bounded auth retry on `401/403`
  - reconnect/backoff after websocket disconnects
- Reused the existing `ConnectionProcessor` / `ExecServerHandler` path
so cloud mode serves the same exec/filesystem RPC surface as local
websocket mode
- Added cloud-specific error variants and minimal docs for the new mode

## Testing
Manual e2e test that fully goes through exec server flow with our codex
cloud agent as orchestrator

codex-rs/exec-server/README.md

@@ -22,6 +22,11 @@ the wire.
 The CLI entrypoint supports:
 
 - `ws://IP:PORT` (default)
+- `--remote URL --executor-id ID [--name NAME]`
+
+Remote mode registers the local exec-server with the executor registry,
+then reconnects to the service-provided rendezvous websocket as the executor.
+It requires a bearer token in `CODEX_EXEC_SERVER_REMOTE_BEARER_TOKEN`.
 
 Wire framing:
 
@@ -308,6 +313,8 @@ The crate exports:
 - `DEFAULT_LISTEN_URL` and `ExecServerListenUrlParseError`
 - `ExecServerRuntimePaths`
 - `run_main()` for embedding the websocket server
+- `RemoteExecutorConfig` and `run_remote_executor()` for embedding remote
+  registration mode
 
 Callers must pass `ExecServerRuntimePaths` to `run_main()`. The top-level
 `codex exec-server` command builds these paths from the `codex` arg0 dispatch