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/thread-manager-sample/src/main.rs

@@ -208,6 +208,7 @@ fn new_config(model: Option<String>, arg0_paths: Arg0DispatchPaths) -> anyhow::R
         hide_agent_reasoning: false,
         show_raw_agent_reasoning: false,
         base_instructions: None,
+        base_instructions_provenance: None,
         developer_instructions: None,
         guardian_policy_config: None,
         include_permissions_instructions: false,