[codex] support executor registry remote environments (#21323)

## 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`

codex-rs/exec-server/Cargo.toml

@@ -26,7 +26,6 @@ futures = { workspace = true }
 reqwest = { workspace = true, features = ["json", "rustls-tls", "stream"] }
 serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }
-sha2 = { workspace = true }
 thiserror = { workspace = true }
 toml = { workspace = true }
 tokio = { workspace = true, features = [