Add previous-session workspace path helpers and layout support

docs/internal/workspace.md

@@ -17,7 +17,8 @@ Outputs:
 
 ## Boundaries
 Owns:
-- Session-level path layout (`inputs/`, `audio/`, `transcripts/`, `artifacts/`, `reports/`, `logs/`, `config/`, `current/`, `runs/`)
+- Session-level path layout (`inputs/`, `audio/`, `transcripts/`, `artifacts/`, `reports/`, `logs/`, `config/`, `current/`, `runs/`, `previous/`)
+  - `previous/manifest.json` and `previous/artifacts/**` are reserved for prepared previous-session state
 - Run-local stage sandbox layout under `runs/{run_id}/{stage}/`
 - Session lock acquisition/release (`.lock`)
 
@@ -65,4 +66,5 @@ None directly in this subsystem. Stages may use object storage adapters and then
 - Session root is campaign-aware: `{workspace.root}/work/{campaign}/{session_id}`.
 - Run roots are always nested: `runs/{run_id}` under the session root.
 - Run-local output promotion must end in canonical session paths.
+- `previous/**` is session-durable state and must not be treated as run-local output scratch state.
 - Cleanup only targets run-scoped directories and must never delete configured root directories.