feat(worker): add wait_for_task_change MCP tool

Replaces the list handler's Start-Sleep + blind get_task poll (Phase 3
of the merge-helper prompt) with a blocking MCP tool that returns as
soon as a task leaves Queued/Running, or times out. Implemented as an
async DB poll (short-lived DbContext, 500ms delay, no held connection)
rather than hooking HubBroadcaster, keeping the existing broadcast
callers untouched. timeoutSeconds is clamped server-side to 170s, under
the list handler's 200s MCP_TOOL_TIMEOUT.

src/ClaudeDo.Data/PromptFiles.cs

@@ -270,7 +270,7 @@ public static class PromptFiles
         - Running or WaitingForChildren → leave it; only poll.
         - WaitingForReview → leave it; it goes straight to Phase 4.
 
-        Poll get_task until every task has left Queued and Running — WaitingForReview on success, Failed on error. Report progress as tasks land; do not poll silently for minutes.
+        Call wait_for_task_change with the ids of every task still Queued or Running (timeoutSeconds up to 170) instead of sleeping and polling get_task yourself. It returns as soon as any of them leaves Queued/Running — WaitingForReview on success, Failed on error — or reports timedOut if none did. Report progress as tasks land, then call it again with whatever ids are still Queued/Running until none remain.
 
         ## Phase 4 — Review and merge
         One task at a time, in the order the brief lists them.