feat: add support for -c/--config to override individual config items

[bolinfest]

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.