Security improvements: per-group session isolation, remove built-in Gmail

- Isolate Claude sessions per-group (data/sessions/{group}/.claude/)
  to prevent cross-group access to conversation history
- Remove Gmail MCP from built-in (now available via /add-gmail skill)
- Add SECURITY.md documenting the security model
- Move docs to docs/ folder (SPEC.md, REQUIREMENTS.md, SECURITY.md)
- Update documentation to reflect changes

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.mcp.json

@@ -1,8 +1,3 @@
 {
-  "mcpServers": {
-    "gmail": {
-      "command": "npx",
-      "args": ["-y", "@gongrzhe/server-gmail-autoauth-mcp"]
-    }
-  }
+  "mcpServers": {}
 }