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/main.rs

@@ -5,6 +5,7 @@ use std::path::PathBuf;
 
 use clap::ArgAction;
 use clap::Parser;
+use codex_core::SandboxModeCliArg;
 use codex_exec::Cli as ExecCli;
 use codex_interactive::Cli as InteractiveCli;
 use codex_repl::Cli as ReplCli;
@@ -70,6 +71,10 @@ struct SeatbeltCommand {
     #[arg(long = "writable-root", short = 'w', value_name = "DIR", action = ArgAction::Append, use_value_delimiter = false)]
     writable_roots: Vec<PathBuf>,
 
+    /// Configure the process restrictions for the command.
+    #[arg(long = "sandbox", short = 's')]
+    sandbox_policy: SandboxModeCliArg,
+
     /// Full command args to run under seatbelt.
     #[arg(trailing_var_arg = true)]
     command: Vec<String>,
@@ -101,9 +106,10 @@ async fn main() -> anyhow::Result<()> {
         Some(Subcommand::Debug(debug_args)) => match debug_args.cmd {
             DebugCommand::Seatbelt(SeatbeltCommand {
                 command,
+                sandbox_policy,
                 writable_roots,
             }) => {
-                seatbelt::run_seatbelt(command, writable_roots).await?;
+                seatbelt::run_seatbelt(command, sandbox_policy.into(), writable_roots).await?;
             }
         },
     }

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()