Enforce workspace boundaries with configurable restriction option

Implemented a unified path validation helper to ensure filesystem operations stay within the designated workspace. This now supports a 'restrict_to_workspace' option in config.json (enabled by default) to allow flexibility for specific environments while maintaining a secure default posture. I've updated read_file, write_file, list_dir, append_file, edit_file, and exec tools to respect this setting and included tests for both restricted and unrestricted modes.

config.example.json

@@ -2,6 +2,7 @@
   "agents": {
     "defaults": {
       "workspace": "~/.picoclaw/workspace",
+      "restrict_to_workspace": true,
       "model": "glm-4.7",
       "max_tokens": 8192,
       "temperature": 0.7,