feat(worker): dependsOn via MCP + honest staleness signal in merge preview

Adds a user/MCP-declared task dependency (DependsOnTaskId) distinct from the
planning chain's internal BlockedByTaskId: add_task/update_task can set it,
the queue picker skips a Queued task until the dependency reaches Done, a
Failed/Cancelled dependency leaves the dependent blocked instead of starving
silently, and setting a link rejects self-reference/unknown-id/cycles.
get_task/list_tasks/batch_get_tasks now report blocked/blockedReason, and
wait_for_task_change reports "Blocked" immediately instead of running out its
timeout on a task the picker will never claim.

preview_merge/preview_merge_set gain staleFiles: files a branch touches that
the target branch also changed since the branch's fork point, a more honest
staleness signal than `behind` alone.

src/ClaudeDo.Worker/State/Interfaces/ITaskStateService.cs

@@ -25,6 +25,10 @@ public interface ITaskStateService
     Task<TransitionResult> BlockOnAsync(string taskId, string predecessorTaskId, CancellationToken ct);
     Task<TransitionResult> UnblockAsync(string taskId, CancellationToken ct);
 
+    // dependsOnTaskId null clears the dependency. Rejects self-reference, an unknown dependency
+    // id, and a link that would create a cycle -- see TaskStateService for the walk.
+    Task<TransitionResult> SetDependsOnAsync(string taskId, string? dependsOnTaskId, CancellationToken ct);
+
     // Surfaces a WaitingForChildren parent for review once all its children are terminal.
     // Best-effort (swallows and logs failures) — safe to call after any child mutation,
     // e.g. deleting the last non-terminal child (no terminal transition fires for a delete).