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/conversation_history_widget.rs

@@ -1,6 +1,7 @@
 use crate::history_cell::CommandOutput;
 use crate::history_cell::HistoryCell;
 use crate::history_cell::PatchEventType;
+use codex_core::config::Config;
 use codex_core::protocol::FileChange;
 use crossterm::event::KeyCode;
 use crossterm::event::KeyEvent;
@@ -181,13 +182,10 @@ impl ConversationHistoryWidget {
         self.add_to_history(HistoryCell::new_patch_event(event_type, changes));
     }
 
-    pub fn add_session_info(
-        &mut self,
-        model: String,
-        cwd: std::path::PathBuf,
-        approval_policy: codex_core::protocol::AskForApproval,
-    ) {
-        self.add_to_history(HistoryCell::new_session_info(model, cwd, approval_policy));
+    /// Note `model` could differ from `config.model` if the agent decided to
+    /// use a different model than the one requested by the user.
+    pub fn add_session_info(&mut self, config: &Config, model: String, cwd: PathBuf) {
+        self.add_to_history(HistoryCell::new_session_info(config, model, cwd));
     }
 
     pub fn add_active_exec_command(&mut self, call_id: String, command: Vec<String>) {