add(core): managed config (#3868)

## Summary

- Factor `load_config_as_toml` into `core::config_loader` so config
loading is reusable across callers.
- Layer `~/.codex/config.toml`, optional `~/.codex/managed_config.toml`,
and macOS managed preferences (base64) with recursive table merging and
scoped threads per source.

## Config Flow

```
Managed prefs (macOS profile: com.openai.codex/config_toml_base64)
                               ▲
                               │
~/.codex/managed_config.toml   │  (optional file-based override)
                               ▲
                               │
                ~/.codex/config.toml (user-defined settings)
```

- The loader searches under the resolved `CODEX_HOME` directory
(defaults to `~/.codex`).
- Managed configs let administrators ship fleet-wide overrides via
device profiles which is useful for enforcing certain settings like
sandbox or approval defaults.
- For nested hash tables: overlays merge recursively. Child tables are
merged key-by-key, while scalar or array values replace the prior layer
entirely. This lets admins add or tweak individual fields without
clobbering unrelated user settings.

codex-rs/core/Cargo.toml

@@ -19,13 +19,13 @@ async-trait = { workspace = true }
 base64 = { workspace = true }
 bytes = { workspace = true }
 chrono = { workspace = true, features = ["serde"] }
+codex-app-server-protocol = { workspace = true }
 codex-apply-patch = { workspace = true }
 codex-file-search = { workspace = true }
 codex-mcp-client = { workspace = true }
-codex-rmcp-client = { workspace = true }
-codex-protocol = { workspace = true }
-codex-app-server-protocol = { workspace = true }
 codex-otel = { workspace = true, features = ["otel"] }
+codex-protocol = { workspace = true }
+codex-rmcp-client = { workspace = true }
 codex-utils-string = { workspace = true }
 dirs = { workspace = true }
 dunce = { workspace = true }
@@ -76,6 +76,9 @@ wildmatch = { workspace = true }
 landlock = { workspace = true }
 seccompiler = { workspace = true }
 
+[target.'cfg(target_os = "macos")'.dependencies]
+core-foundation = "0.9"
+
 # Build OpenSSL from source for musl builds.
 [target.x86_64-unknown-linux-musl.dependencies]
 openssl-sys = { workspace = true, features = ["vendored"] }