Add executor-local config reads to the exec server (#37408)

## What changed

- Add the `environmentConfig/read` RPC for selecting literal TOML paths from executor-local config and requirements layers.
- Return layer precedence, cloud insertion points, source and base-directory metadata, and executor home and hostname context without normalizing path-bearing values.
- Expose the operation through both remote clients and local `Environment` instances, with invalid selectors reported as invalid parameters.

## Testing

- Cover projected remote config reads and rejection of empty selectors.

GitOrigin-RevId: fee15bf833de6bcb2058d405ed5b1d5d928d7218

codex-rs/Cargo.lock

@@ -2994,6 +2994,7 @@ dependencies = [
  "bytes",
  "clatter",
  "codex-api",
+ "codex-config",
  "codex-exec-server-protocol",
  "codex-exec-server-test-support",
  "codex-file-system",
@@ -3004,11 +3005,13 @@ dependencies = [
  "codex-sandboxing",
  "codex-test-binary-support",
  "codex-utils-absolute-path",
+ "codex-utils-home-dir",
  "codex-utils-path-uri",
  "codex-utils-pty",
  "codex-utils-rustls-provider",
  "codex-websocket-client",
  "ctor 0.6.3",
+ "dirs",
  "futures",
  "http 1.4.0",
  "libc",