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/debug_sandbox.rs

@@ -1,5 +1,6 @@
 use std::path::PathBuf;
 
+use codex_common::CliConfigOverrides;
 use codex_common::SandboxPermissionOption;
 use codex_core::config::Config;
 use codex_core::config::ConfigOverrides;
@@ -20,12 +21,14 @@ pub async fn run_command_under_seatbelt(
     let SeatbeltCommand {
         full_auto,
         sandbox,
+        config_overrides,
         command,
     } = command;
     run_command_under_sandbox(
         full_auto,
         sandbox,
         command,
+        config_overrides,
         codex_linux_sandbox_exe,
         SandboxType::Seatbelt,
     )
@@ -39,12 +42,14 @@ pub async fn run_command_under_landlock(
     let LandlockCommand {
         full_auto,
         sandbox,
+        config_overrides,
         command,
     } = command;
     run_command_under_sandbox(
         full_auto,
         sandbox,
         command,
+        config_overrides,
         codex_linux_sandbox_exe,
         SandboxType::Landlock,
     )
@@ -60,16 +65,22 @@ async fn run_command_under_sandbox(
     full_auto: bool,
     sandbox: SandboxPermissionOption,
     command: Vec<String>,
+    config_overrides: CliConfigOverrides,
     codex_linux_sandbox_exe: Option<PathBuf>,
     sandbox_type: SandboxType,
 ) -> anyhow::Result<()> {
     let sandbox_policy = create_sandbox_policy(full_auto, sandbox);
     let cwd = std::env::current_dir()?;
-    let config = Config::load_with_overrides(ConfigOverrides {
-        sandbox_policy: Some(sandbox_policy),
-        codex_linux_sandbox_exe,
-        ..Default::default()
-    })?;
+    let config = Config::load_with_cli_overrides(
+        config_overrides
+            .parse_overrides()
+            .map_err(anyhow::Error::msg)?,
+        ConfigOverrides {
+            sandbox_policy: Some(sandbox_policy),
+            codex_linux_sandbox_exe,
+            ..Default::default()
+        },
+    )?;
     let stdio_policy = StdioPolicy::Inherit;
     let env = create_env(&config.shell_environment_policy);