Add remote session loading

docs/internal/stage-prepare.md

@@ -11,6 +11,7 @@ Prepare owns:
 ## Inputs and outputs
 Inputs:
 - resolved config/campaign/session (`pipeline.yml`, `campaign.yml`, `session.yml`);
+- remote session provenance when `session.yml` was loaded from S3;
 - campaign or session input files (`speakers`, `autocorrect`, `glossary`);
 - audio source:
   - local: `session.inputs.audio_dir` or `session.inputs.audio_files`;
@@ -72,6 +73,7 @@ Does not own:
 ## State and manifest behavior
 - Ensures workspace layout exists.
 - Materializes canonical input files and audio files.
+- Records `inputs/session.yml` provenance as local `session_config` or remote `session_config.s3`.
 - Resolves campaign-provided stable input paths relative to `campaign.yml`.
 - Resolves session-provided stable input overrides relative to `session.yml`.
 - Scans enabled configured artifact inputs for canonical sources: