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/tui/src/app.rs

@@ -4,10 +4,9 @@ use crate::git_warning_screen::GitWarningOutcome;
 use crate::git_warning_screen::GitWarningScreen;
 use crate::scroll_event_helper::ScrollEventHelper;
 use crate::tui;
-use codex_core::protocol::AskForApproval;
+use codex_core::config::Config;
 use codex_core::protocol::Event;
 use codex_core::protocol::Op;
-use codex_core::protocol::SandboxPolicy;
 use color_eyre::eyre::Result;
 use crossterm::event::KeyCode;
 use crossterm::event::KeyEvent;
@@ -34,12 +33,10 @@ pub(crate) struct App<'a> {
 
 impl App<'_> {
     pub(crate) fn new(
-        approval_policy: AskForApproval,
-        sandbox_policy: SandboxPolicy,
+        config: Config,
         initial_prompt: Option<String>,
         show_git_warning: bool,
         initial_images: Vec<std::path::PathBuf>,
-        model: Option<String>,
         disable_response_storage: bool,
     ) -> Self {
         let (app_event_tx, app_event_rx) = channel();
@@ -80,12 +77,10 @@ impl App<'_> {
         }
 
         let chat_widget = ChatWidget::new(
-            approval_policy,
-            sandbox_policy,
+            config,
             app_event_tx.clone(),
             initial_prompt.clone(),
             initial_images,
-            model,
             disable_response_storage,
         );