Add archive storage path configuration

docs/runbooks/s3-archive-foundations.md

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+# S3 Archive Foundations Runbook
+
+This runbook documents the currently implemented storage/archive foundations and the boundaries of current behavior.
+
+## Implemented Now
+
+- config modeling for:
+  - `pipeline.storage.s3`
+  - `pipeline.spool`
+  - `pipeline.archive`
+  - `session.inputs.audio_s3`
+- promotion rule validation for safe relative paths
+- run ID generation and path/key helper functions
+- manifest run/path identity fields
+
+## Not Implemented Yet
+
+- real S3 backend integration
+- AWS SDK wiring
+- prepare-stage S3 object listing or download
+- archive-stage S3 upload or promotion writes
+- writing `current/manifest.json` or `current/run_id.txt` in S3
+
+## Operational Notes
+
+- local audio workflows remain the active development path (`audio_dir` or `audio_files`)
+- `audio_s3` and local audio config are mutually exclusive
+- do not place AWS credentials in Narratio config files
+
+## Next Implementation Target
+
+Build the storage backend layer that can list/download/upload S3 objects through a fake-tested interface, then wire prepare/archive stages to use that backend.