Expose parentTaskNumber in task_update and task_patch #245

MCP surface for #243. Depends on #244.

Re-read TaskTools.cs before editing. It was being rewritten in parallel when this was planned — proposal wiring, plus explanation and review parameters appended to the very signatures this task changes. A stale read already cost one failed attempt.

Work

• task_update: int? parentTaskNumber with no default, placed before every defaulted parameter so the MCP schema marks it required. Note the ordering conflict: the parallel work appends optional parameters to the same list.
• task_patch: parse parentTaskNumber in ParsePatch — accept null or a positive integer, reject everything else with a clear message. Extend ParsedTaskPatch.
• Resolve the number to an id through a shared helper; task_create can reuse it.
• Return the new parent: ProjectTaskView.ParentTaskNumber is stale after a move in both tools, so the response would otherwise report the old parent.

Tool descriptions

Both need to state the new contract — required and echoed for task_update, null means top level for task_patch.

McpDiscoveryTests.cs:60 pins three phrases in the task_patch description that must survive the rewrite: "Omitted fields are preserved", "assignee null clears", "task_update remains the full replacement".