feat: harden runtime readiness and session workflow

harness/tht/session/store.py

@@ -242,6 +242,25 @@ def close_session(session_id: str, sessions_root: Path) -> SessionManifest:
     return manifest
 
 
+def fail_session(session_id: str, sessions_root: Path) -> SessionManifest:
+    """Record a fatal managed-runtime failure without losing phase artifacts."""
+    manifest = load_session(session_id, sessions_root)
+    manifest.status = "failed"
+    manifest.updated_at = datetime.now(UTC)
+    manifest.updated_by = current_author()
+    manifest.to_yaml(sessions_root / session_id / MANIFEST)
+    return manifest
+
+def reopen_session(session_id: str, sessions_root: Path) -> SessionManifest:
+    """Mark a manually resumed session active again."""
+    manifest = load_session(session_id, sessions_root)
+    manifest.status = "open"
+    manifest.updated_at = datetime.now(UTC)
+    manifest.updated_by = current_author()
+    manifest.to_yaml(sessions_root / session_id / MANIFEST)
+    return manifest
+
+
 def _save_touched(manifest: SessionManifest, sessions_root: Path) -> SessionManifest:
     """Persist `manifest` updating updated_at/updated_by (single save path for mutations)."""
     manifest.updated_at = datetime.now(UTC)