* fix(mcp): validate UUID inputs at engine level and add sync_retain tool (#888)
- Add UUID validation in memory_engine for get_memory_unit, delete_memory_unit,
get_mental_model, delete_mental_model, get_mental_model_history (raises ValueError)
- Catch ValueError → 400 in HTTP route handlers
- Add sync_retain MCP tool that calls retain_batch_async directly for immediate
availability (no polling needed)
- Register sync_retain in _ALL_TOOLS, _SINGLE_BANK_TOOLS, UI MCP_TOOL_GROUPS
- Add code-review check for MCP tool registration completeness
* fix: remove UUID validation for mental model IDs (column is TEXT, not UUID)
Mental model IDs are TEXT columns that accept arbitrary string IDs
(e.g., 'team-communication-preferences'). UUID validation was incorrectly
added to get_mental_model, delete_mental_model, and get_mental_model_history.
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