Add remote session loading

docs/internal/artifacts.md

@@ -16,6 +16,7 @@ Outputs:
 - resolved artifact path + provenance (`ResolvedSessionArtifact`);
 - runtime catalog entries for built-ins and configured artifacts;
 - requirement sets for canonical previous-session inputs.
+- canonical S3 session, run, current, session config, audio, and promoted artifact keys.
 
 ## Boundaries
 Owns:
@@ -43,6 +44,14 @@ Does not own:
 - configured artifact: `narratio.artifact.<artifact_key>`
 - canonical previous-session artifact: `narratio.previous_session.artifact.<artifact_key>`
 
+## S3 key helpers
+- session prefix: `{root_prefix}/campaigns/{campaign}/sessions/{session_id}/`
+- session config: `{session_prefix}/session.yml`
+- run prefix: `{session_prefix}/runs/{run_id}/`
+- audio prefix: `{session_prefix}/{session.inputs.audio_s3.prefix}`
+- current manifest: `{session_prefix}/current/manifest.json`
+- current run pointer: `{session_prefix}/current/run_id.txt`
+
 ## Runtime catalog model
 Catalog entries track:
 - `planned`: source is registered for this run;