Add support for custom base instructions (#1645)

Allows providing custom instructions file as a config parameter and
custom instruction text via MCP tool call.

codex-rs/tui/src/lib.rs

@@ -75,6 +75,7 @@ pub fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> std::io::
             model_provider: None,
             config_profile: cli.config_profile.clone(),
             codex_linux_sandbox_exe,
+            base_instructions: None,
         };
         // Parse `-c` overrides from the CLI.
         let cli_kv_overrides = match cli.config_overrides.parse_overrides() {