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/core/src/codex.rs

@@ -36,7 +36,6 @@ use crate::exec::process_exec_tool_call;
 use crate::exec::ExecParams;
 use crate::exec::ExecToolCallOutput;
 use crate::exec::SandboxType;
-use crate::flags::OPENAI_DEFAULT_MODEL;
 use crate::flags::OPENAI_STREAM_MAX_RETRIES;
 use crate::models::ContentItem;
 use crate::models::FunctionCallOutputPayload;
@@ -486,7 +485,6 @@ async fn submission_loop(
                 sandbox_policy,
                 disable_response_storage,
             } => {
-                let model = model.unwrap_or_else(|| OPENAI_DEFAULT_MODEL.to_string());
                 info!(model, "Configuring session");
                 let client = ModelClient::new(model.clone());