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/history_cell.rs

@@ -1,4 +1,5 @@
 use codex_ansi_escape::ansi_escape_line;
+use codex_core::config::Config;
 use codex_core::protocol::FileChange;
 use ratatui::prelude::*;
 use ratatui::style::Color;
@@ -144,9 +145,9 @@ impl HistoryCell {
     }
 
     pub(crate) fn new_session_info(
+        config: &Config,
         model: String,
         cwd: std::path::PathBuf,
-        approval_policy: codex_core::protocol::AskForApproval,
     ) -> Self {
         let mut lines: Vec<Line<'static>> = Vec::new();
 
@@ -158,7 +159,11 @@ impl HistoryCell {
         ]));
         lines.push(Line::from(vec![
             "↳ approval: ".bold(),
-            format!("{:?}", approval_policy).into(),
+            format!("{:?}", config.approval_policy).into(),
+        ]));
+        lines.push(Line::from(vec![
+            "↳ sandbox: ".bold(),
+            format!("{:?}", config.sandbox_policy).into(),
         ]));
         lines.push(Line::from(""));