feat: load defaults into Config and introduce ConfigOverrides (#677)

This changes how instantiating `Config` works and also adds
`approval_policy` and `sandbox_policy` as fields. The idea is:

* All fields of `Config` have appropriate default values.
* `Config` is initially loaded from `~/.codex/config.toml`, so values in
`config.toml` will override those defaults.
* Clients must instantiate `Config` via
`Config::load_with_overrides(ConfigOverrides)` where `ConfigOverrides`
has optional overrides that are expected to be settable based on CLI
flags.

The `Config` should be defined early in the program and then passed
down. Now functions like `init_codex()` take fewer individual parameters
because they can just take a `Config`.

Also, `Config::load()` used to fail silently if `~/.codex/config.toml`
had a parse error and fell back to the default config. This seemed
really bad because it wasn't clear why the values in my `config.toml`
weren't getting picked up. I changed things so that
`load_with_overrides()` returns `Result<Config>` and verified that the
various CLIs print a reasonable error if `config.toml` is malformed.

Finally, I also updated the TUI to show which **sandbox** value is being
used, as we do for other key values like **model** and **approval**.
This was also a reminder that the various values of `--sandbox` are
honored on Linux but not macOS today, so I added some TODOs about fixing
that.

codex-rs/core/tests/live_agent.rs

@@ -17,7 +17,7 @@
 
 use std::time::Duration;
 
-use codex_core::protocol::AskForApproval;
+use codex_core::config::Config;
 use codex_core::protocol::EventMsg;
 use codex_core::protocol::InputItem;
 use codex_core::protocol::Op;
@@ -47,13 +47,14 @@ async fn spawn_codex() -> Codex {
 
     let agent = Codex::spawn(std::sync::Arc::new(Notify::new())).unwrap();
 
+    let config = Config::load_default_config_for_test();
     agent
         .submit(Submission {
             id: "init".into(),
             op: Op::ConfigureSession {
-                model: None,
+                model: config.model,
                 instructions: None,
-                approval_policy: AskForApproval::OnFailure,
+                approval_policy: config.approval_policy,
                 sandbox_policy: SandboxPolicy::NetworkAndFileWriteRestricted,
                 disable_response_storage: false,
             },

codex-rs/core/tests/previous_response_id.rs

@@ -1,6 +1,6 @@
 use std::time::Duration;
 
-use codex_core::protocol::AskForApproval;
+use codex_core::config::Config;
 use codex_core::protocol::InputItem;
 use codex_core::protocol::Op;
 use codex_core::protocol::SandboxPolicy;
@@ -87,13 +87,14 @@ async fn keeps_previous_response_id_between_tasks() {
     let codex = Codex::spawn(std::sync::Arc::new(tokio::sync::Notify::new())).unwrap();
 
     // Init session
+    let config = Config::load_default_config_for_test();
     codex
         .submit(Submission {
             id: "init".into(),
             op: Op::ConfigureSession {
-                model: None,
+                model: config.model,
                 instructions: None,
-                approval_policy: AskForApproval::OnFailure,
+                approval_policy: config.approval_policy,
                 sandbox_policy: SandboxPolicy::NetworkAndFileWriteRestricted,
                 disable_response_storage: false,
             },

codex-rs/core/tests/stream_no_completed.rs

@@ -3,7 +3,7 @@
 
 use std::time::Duration;
 
-use codex_core::protocol::AskForApproval;
+use codex_core::config::Config;
 use codex_core::protocol::InputItem;
 use codex_core::protocol::Op;
 use codex_core::protocol::SandboxPolicy;
@@ -70,13 +70,14 @@ async fn retries_on_early_close() {
 
     let codex = Codex::spawn(std::sync::Arc::new(tokio::sync::Notify::new())).unwrap();
 
+    let config = Config::load_default_config_for_test();
     codex
         .submit(Submission {
             id: "init".into(),
             op: Op::ConfigureSession {
-                model: None,
+                model: config.model,
                 instructions: None,
-                approval_policy: AskForApproval::OnFailure,
+                approval_policy: config.approval_policy,
                 sandbox_policy: SandboxPolicy::NetworkAndFileWriteRestricted,
                 disable_response_storage: false,
             },