fix(mem0-plugin): graceful degradation without API key, stdin support

- SessionStart shows "Mem0 Inactive" banner with identity info when no
  API key set, instead of silently exiting with zero output
- on_user_prompt.sh fires error/file-path detection even without API key
  (local pattern matching, no API needed). Search rubric still gated.
- parse_export_file.py supports '-' argument for stdin piping

mem0-plugin/scripts/parse_export_file.py

@@ -131,8 +131,11 @@ def main() -> None:
 
     filepath = sys.argv[1]
     try:
-        with open(filepath, encoding="utf-8", errors="replace") as f:
-            content = f.read()
+        if filepath == "-":
+            content = sys.stdin.read()
+        else:
+            with open(filepath, encoding="utf-8", errors="replace") as f:
+                content = f.read()
     except OSError as e:
         print(f"Error reading file: {e}", file=sys.stderr)
         print("[]")