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/cli/src/seatbelt.rs

@@ -1,11 +1,13 @@
 use codex_core::exec::create_seatbelt_command;
+use codex_core::protocol::SandboxPolicy;
 use std::path::PathBuf;
 
 pub(crate) async fn run_seatbelt(
     command: Vec<String>,
+    sandbox_policy: SandboxPolicy,
     writable_roots: Vec<PathBuf>,
 ) -> anyhow::Result<()> {
-    let seatbelt_command = create_seatbelt_command(command, &writable_roots);
+    let seatbelt_command = create_seatbelt_command(command, sandbox_policy, &writable_roots);
     let status = tokio::process::Command::new(seatbelt_command[0].clone())
         .args(&seatbelt_command[1..])
         .spawn()