Add realtime start instructions config override (#14270)

- add `realtime_start_instructions` config support
- thread it into realtime context updates, schema, docs, and tests

codex-rs/core/src/config/mod.rs

@@ -470,6 +470,10 @@ pub struct Config {
     /// context appended to websocket session instructions. An empty string
     /// disables startup context injection entirely.
     pub experimental_realtime_ws_startup_context: Option<String>,
+    /// Experimental / do not use. Replaces the built-in realtime start
+    /// instructions inserted into developer messages when realtime becomes
+    /// active.
+    pub experimental_realtime_start_instructions: Option<String>,
     /// When set, restricts ChatGPT login to a specific workspace identifier.
     pub forced_chatgpt_workspace_id: Option<String>,
 
@@ -1241,6 +1245,10 @@ pub struct ConfigToml {
     /// context appended to websocket session instructions. An empty string
     /// disables startup context injection entirely.
     pub experimental_realtime_ws_startup_context: Option<String>,
+    /// Experimental / do not use. Replaces the built-in realtime start
+    /// instructions inserted into developer messages when realtime becomes
+    /// active.
+    pub experimental_realtime_start_instructions: Option<String>,
     pub projects: Option<HashMap<String, ProjectConfig>>,
 
     /// Controls the web search tool mode: disabled, cached, or live.
@@ -2426,6 +2434,7 @@ impl Config {
             experimental_realtime_ws_model: cfg.experimental_realtime_ws_model,
             experimental_realtime_ws_backend_prompt: cfg.experimental_realtime_ws_backend_prompt,
             experimental_realtime_ws_startup_context: cfg.experimental_realtime_ws_startup_context,
+            experimental_realtime_start_instructions: cfg.experimental_realtime_start_instructions,
             forced_chatgpt_workspace_id,
             forced_login_method,
             include_apply_patch_tool: include_apply_patch_tool_flag,