feat(mcp): paths filter for get_task_diff, subset hint for preview_merge_set

get_task_diff gained an optional paths param (git pathspec, both stat and
full-diff modes) so a large multi-file task can be narrowed to the files
actually in question instead of shipping the whole diff.

preview_merge_set now also reports Subsets: pairs where one task's changed
files are a proper subset of another's in the same set, the strongest
available post-hoc signal that a task may be redundant with another.

src/ClaudeDo.Worker/Runner/WorktreeManager.cs

@@ -147,7 +147,7 @@ public sealed class WorktreeManager
         await _git.CommitAsync(ctx.WorktreePath, message, ct);
 
         var head = await _git.RevParseHeadAsync(ctx.WorktreePath, ct);
-        var diffStat = await _git.DiffStatAsync(ctx.WorktreePath, ctx.BaseCommit, head, ct);
+        var diffStat = await _git.DiffStatAsync(ctx.WorktreePath, ctx.BaseCommit, head, ct: ct);
 
         using var context = _dbFactory.CreateDbContext();
         var wtRepo = new WorktreeRepository(context);