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/exec/src/cli.rs

@@ -1,5 +1,6 @@
 use clap::Parser;
 use clap::ValueEnum;
+use codex_common::CliConfigOverrides;
 use codex_common::SandboxPermissionOption;
 use std::path::PathBuf;
 
@@ -33,9 +34,8 @@ pub struct Cli {
     #[arg(long = "skip-git-repo-check", default_value_t = false)]
     pub skip_git_repo_check: bool,
 
-    /// Disable server‑side response storage (sends the full conversation context with every request)
-    #[arg(long = "disable-response-storage", default_value_t = false)]
-    pub disable_response_storage: bool,
+    #[clap(skip)]
+    pub config_overrides: CliConfigOverrides,
 
     /// Specifies color settings for use in the output.
     #[arg(long = "color", value_enum, default_value_t = Color::Auto)]