feat: add support for -c/--config to override individual config items (#1137)

This PR introduces support for `-c`/`--config` so users can override
individual config values on the command line using `--config
name=value`. Example:

```
codex --config model=o4-mini
```

Making it possible to set arbitrary config values on the command line
results in a more flexible configuration scheme and makes it easier to
provide single-line examples that can be copy-pasted from documentation.

Effectively, it means there are four levels of configuration for some
values:

- Default value (e.g., `model` currently defaults to `o4-mini`)
- Value in `config.toml` (e.g., user could override the default to be
`model = "o3"` in their `config.toml`)
- Specifying `-c` or `--config` to override `model` (e.g., user can
include `-c model=o3` in their list of args to Codex)
- If available, a config-specific flag can be used, which takes
precedence over `-c` (e.g., user can specify `--model o3` in their list
of args to Codex)

Now that it is possible to specify anything that could be configured in
`config.toml` on the command line using `-c`, we do not need to have a
custom flag for every possible config option (which can clutter the
output of `--help`). To that end, as part of this PR, we drop support
for the `--disable-response-storage` flag, as users can now specify `-c
disable_response_storage=true` to get the equivalent functionality.

Under the hood, this works by loading the `config.toml` into a
`toml::Value`. Then for each `key=value`, we create a small synthetic
TOML file with `value` so that we can run the TOML parser to get the
equivalent `toml::Value`. We then parse `key` to determine the point in
the original `toml::Value` to do the insert/replace. Once all of the
overrides from `-c` args have been applied, the `toml::Value` is
deserialized into a `ConfigToml` and then the `ConfigOverrides` are
applied, as before.

codex-rs/cli/src/proto.rs

@@ -2,6 +2,7 @@ use std::io::IsTerminal;
 use std::sync::Arc;
 
 use clap::Parser;
+use codex_common::CliConfigOverrides;
 use codex_core::Codex;
 use codex_core::config::Config;
 use codex_core::config::ConfigOverrides;
@@ -13,9 +14,12 @@ use tracing::error;
 use tracing::info;
 
 #[derive(Debug, Parser)]
-pub struct ProtoCli {}
+pub struct ProtoCli {
+    #[clap(skip)]
+    pub config_overrides: CliConfigOverrides,
+}
 
-pub async fn run_main(_opts: ProtoCli) -> anyhow::Result<()> {
+pub async fn run_main(opts: ProtoCli) -> anyhow::Result<()> {
     if std::io::stdin().is_terminal() {
         anyhow::bail!("Protocol mode expects stdin to be a pipe, not a terminal");
     }
@@ -24,7 +28,12 @@ pub async fn run_main(_opts: ProtoCli) -> anyhow::Result<()> {
         .with_writer(std::io::stderr)
         .init();
 
-    let config = Config::load_with_overrides(ConfigOverrides::default())?;
+    let ProtoCli { config_overrides } = opts;
+    let overrides_vec = config_overrides
+        .parse_overrides()
+        .map_err(anyhow::Error::msg)?;
+
+    let config = Config::load_with_cli_overrides(overrides_vec, ConfigOverrides::default())?;
     let ctrl_c = notify_on_sigint();
     let (codex, _init_id) = Codex::spawn(config, ctrl_c.clone()).await?;
     let codex = Arc::new(codex);