CVE-2026-55527: praisonaiagents vulnerable to arbitrary file write via unsanitized `user_id` in `FileMemory.__init__()` — path traversal to any writable location

praisonaiagents/memory/file_memory.py::FileMemory.__init__() constructs all memory file paths by directly joining the user_id parameter to a base path:

self.user_path = self.base_path / user_id      # LINE 145 — no sanitization

No validation or normalization is applied to user_id before the path join. An attacker who can supply a user_id containing ../ sequences can write arbitrary JSON files (memory content) to any writable location on the filesystem.

The vulnerability is confirmed live on the current main branch (praisonaiagents==1.6.52) and is distinct from GHSA-766v-q9x3-g744 (which covered MultiAgentMonitor in an example file, not FileMemory in the core library).