Partial task updates shipped #203

Task #119 and subtasks #125-#127 were completed. Conderis now provides task_patch with presence-aware semantics, preserved legacy task_update behavior, atomic validation, and no redundant post-write task reads.

Commit f46501b was deployed. Focused MCP tools/list and tools/call tests passed 4/4. A production description-only task_patch updated #105 from version 1 to 2 while preserving its title, status, priority, and exact assignee reference. Assignment validation now reuses the transaction DbContext. Nested read contexts in RelationService were identified as a separate lower-risk performance concern and intentionally left outside #119.