Preserve base instruction provenance across sessions (#37446)

## Why

Persisted base instructions need to retain whether they were explicitly customized or generated from a model template. Without that distinction, forks and config-lock replays can treat model-generated instructions as custom and keep the wrong template after a model or personality change.

## What changed

- Record custom or model provenance with base instructions in rollouts and config locks.
- Preserve custom instructions across model changes, while allowing model-generated instructions to follow the selected model and personality.
- Keep legacy rollouts without provenance compatible and propagate provenance through forks and spawned agents.

## Testing

- Cover provenance serialization and legacy rollout decoding.
- Cover role personality changes, forked model changes, config-lock replay, and multi-agent configuration.

GitOrigin-RevId: 801c26559835f558027ce5112700c0cd31fcb21b

codex-rs/app-server/src/external_agent_migration/session_importer.rs

@@ -26,6 +26,7 @@ use codex_external_agent_migration::sessions::record_completed_session_imports;
 use codex_models_manager::manager::RefreshStrategy;
 use codex_protocol::ThreadId;
 use codex_protocol::models::BaseInstructions;
+use codex_protocol::models::BaseInstructionsProvenance;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::MultiAgentVersion;
 use codex_protocol::protocol::RolloutItem;
@@ -448,6 +449,17 @@ impl ExternalAgentSessionImporter {
                     .base_instructions
                     .clone()
                     .unwrap_or_else(|| model_info.get_model_instructions(config.personality)),
+                provenance: Some(config.base_instructions_provenance.clone().unwrap_or_else(
+                    || {
+                        if config.base_instructions.is_some() {
+                            BaseInstructionsProvenance::Custom
+                        } else {
+                            BaseInstructionsProvenance::Model {
+                                model: model_info.slug.clone(),
+                            }
+                        }
+                    },
+                )),
             },
             dynamic_tools: Vec::new(),
             selected_capability_roots: Vec::new(),

codex-rs/app-server/src/request_processors/feedback_processor.rs

@@ -624,6 +624,7 @@ mod tests {
                     cwd: tempdir.path().to_path_buf(),
                     base_instructions: Some(codex_protocol::models::BaseInstructions {
                         text: "actual developer prompt".to_string(),
+                        provenance: None,
                     }),
                     ..Default::default()
                 },