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.Data/Git/GitService.cs

@@ -389,6 +389,21 @@ public sealed class GitService
             .Count(s => s.Length > 0);
     }
 
+    /// <summary>Files that differ between two exact refs (2-dot, no merge-base resolution) -- used to see
+    /// what a target branch itself picked up since a task's fork point, as opposed to
+    /// <see cref="CountChangedFilesAsync"/>'s 3-dot count of a branch's own changes.</summary>
+    public async Task<IReadOnlyList<string>> GetChangedFileNamesAsync(
+        string repoDir, string fromRef, string toRef, CancellationToken ct = default)
+    {
+        var (exitCode, stdout, _) = await RunGitAsync(repoDir,
+            ["diff", "--name-only", $"{fromRef}..{toRef}"], ct);
+        if (exitCode != 0) return Array.Empty<string>();
+        return stdout
+            .Split('\n', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries)
+            .Where(s => s.Length > 0)
+            .ToList();
+    }
+
     private static async Task<(int ExitCode, string Stdout, string Stderr)> RunGitAsync(
         string workDir, IEnumerable<string> args, CancellationToken ct, string? stdinData = null, bool trimOutput = true)
     {