Implement atomic partial task updates with presence-aware binding #126

Delivered

• Added additive task_patch; legacy task_update remains replacement-style.
• Raw JSON object binding distinguishes omitted, null, empty, and supplied values.
• Description-only patches preserve title, status, priority, and even an inactive historical assignee without re-resolution.
• Explicit assignment changes resolve authorized stable references; null clears assignment.
• Validation, permission, parent/subtask, and optimistic-concurrency failures perform zero writes.
• Empty patches validate write permission and exact version without changing version, timestamp, or emitting a write event.
• Patch writes and Markdown relation reconciliation commit atomically.
• Removed a redundant post-commit database reread that could briefly contend on SQLite.

Focused MCP integration tests: 2 passed, 0 failed.