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/core/src/session/mod.rs

@@ -103,6 +103,7 @@ use codex_protocol::items::UserMessageItem;
 use codex_protocol::models::ActivePermissionProfile;
 use codex_protocol::models::AdditionalPermissionProfile;
 use codex_protocol::models::BaseInstructions;
+use codex_protocol::models::BaseInstructionsProvenance;
 use codex_protocol::models::PermissionProfile;
 use codex_protocol::models::SandboxEnforcement;
 use codex_protocol::openai_models::ModelInfo;
@@ -1263,6 +1264,7 @@ impl Session {
         let state = self.state.lock().await;
         BaseInstructions {
             text: state.session_configuration.base_instructions.clone(),
+            provenance: state.base_instructions_provenance.clone(),
         }
     }