feat: add collab prompt (#9208)

Adding a prompt for collab tools. This is only for internal use and the
prompt won't be gated for now as it is not stable yet.

The goal of this PR is to provide the tool required to iterate on the
prompt

codex-rs/core/src/codex.rs

@@ -539,12 +539,24 @@ impl Session {
             features: &per_turn_config.features,
         });
 
+        let base_instructions = if per_turn_config.features.enabled(Feature::Collab) {
+            const COLLAB_INSTRUCTIONS: &str =
+                include_str!("../templates/collab/experimental_prompt.md");
+            let base = session_configuration
+                .base_instructions
+                .as_deref()
+                .unwrap_or(model_info.base_instructions.as_str());
+            Some(format!("{base}\n\n{COLLAB_INSTRUCTIONS}"))
+        } else {
+            session_configuration.base_instructions.clone()
+        };
+
         TurnContext {
             sub_id,
             client,
             cwd: session_configuration.cwd.clone(),
             developer_instructions: session_configuration.developer_instructions.clone(),
-            base_instructions: session_configuration.base_instructions.clone(),
+            base_instructions,
             compact_prompt: session_configuration.compact_prompt.clone(),
             user_instructions: session_configuration.user_instructions.clone(),
             approval_policy: session_configuration.approval_policy.value(),