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/Queue/QueuePicker.cs

@@ -15,7 +15,10 @@ public sealed class QueuePicker : IQueuePicker
     {
         // Atomic queue claim: UPDATE + RETURNING in a single statement prevents TOCTOU races.
         // Raw SQL because EF cannot express UPDATE...RETURNING.
-        // Eligible task must be Queued, unblocked, not manual, and due (or unscheduled).
+        // Eligible task must be Queued, unblocked (chain and depends-on), not manual, and due
+        // (or unscheduled). depends_on_task_id only gates on the dependency's Status='done' --
+        // unlike blocked_by_task_id there is no cascade on the dependency failing, so a task
+        // with a Failed dependency simply stays here, skipped, until someone intervenes.
         // EF SQLite stores DateTime as "yyyy-MM-dd HH:mm:ss.fffffff" — same format used here for comparison.
         await using var ctx = await _dbFactory.CreateDbContextAsync(ct);
         var nowStr = now.ToUniversalTime().ToString("yyyy-MM-dd HH:mm:ss.fffffff");
@@ -29,6 +32,11 @@ public sealed class QueuePicker : IQueuePicker
                   AND t.blocked_by_task_id IS NULL
                   AND t.is_manual = 0
                   AND (t.scheduled_for IS NULL OR t.scheduled_for <= {0})
+                  AND (t.depends_on_task_id IS NULL
+                       OR EXISTS (
+                           SELECT 1 FROM tasks d
+                           WHERE d.id = t.depends_on_task_id AND d.status = 'done'
+                       ))
                 ORDER BY t.sort_order ASC, t.created_at ASC
                 LIMIT 1
             )