fix(mem0-plugin): remove userConfig from plugin.json, add auto-deps install

- Remove userConfig from .claude-plugin/plugin.json — OAuth flow handles
  auth via mcp.mem0.ai, userConfig prompt was redundant and confusing
- Add ensure_deps.sh to install mem0ai SDK into persistent plugin venv
- Add requirements.txt (mem0ai) for dependency tracking
- Wire SessionStart hook to auto-install deps on first run
- Update setup_coding_categories.py to find venv site-packages
- Update onboarding SKILL.md with venv-aware install steps

mem0-plugin/hooks/hooks.json

@@ -1,6 +1,16 @@
 {
   "hooks": {
     "SessionStart": [
+      {
+        "hooks": [
+          {
+            "type": "command",
+            "command": "diff -q \"${CLAUDE_PLUGIN_ROOT}/requirements.txt\" \"${CLAUDE_PLUGIN_DATA}/requirements.txt\" >/dev/null 2>&1 || \"${CLAUDE_PLUGIN_ROOT}/scripts/ensure_deps.sh\"",
+            "statusMessage": "Installing mem0 SDK...",
+            "timeout": 60
+          }
+        ]
+      },
       {
         "matcher": "startup|resume|compact",
         "hooks": [