## Summary
Support registry-backed remote executors end to end so downstream
services can resolve an executor id into an exec-server URL and make
that environment available to Codex without relying on the legacy cloud
environments flow.
## What changed
- switch remote executor registration to the executor registry bootstrap
contract
- allow named remote environments to be inserted into
`EnvironmentManager` at runtime
- add the experimental app-server RPC `environment/add` so initialized
experimental clients can register those remote environments for later
`thread/start` and `turn/start` selection
## Validation
Ran focused validation locally:
- `cargo test -p codex-exec-server environment_manager_`
- `cargo test -p codex-exec-server
register_executor_posts_with_bearer_token_header`
- `cargo test -p codex-app-server-protocol`
