refactor(merge): drop dead hunks conflict API

GetConflictsAsync/GetMergeConflicts (+ MergeConflicts/ConflictFileContent/ConflictFileDto/ConflictHunkDto DTOs and the now-orphaned GitService.ShowStageAsync) were superseded by the segment-based GetMergeConflictDocuments path and had no production callers. Removes the IWorkerClient member, both test fakes, the lingering test, and updates the Worker/Ui/Data CLAUDE.md surface notes.

src/ClaudeDo.Data/Git/GitService.cs

@@ -280,17 +280,6 @@ public sealed class GitService
             .ToList();
     }
 
-    /// <summary>
-    /// Reads a conflicted file's blob at a merge stage: 1=base, 2=ours, 3=theirs.
-    /// Returns null when the stage doesn't exist (e.g. add/add conflict has no base).
-    /// Output is NOT trimmed so file content round-trips exactly.
-    /// </summary>
-    public async Task<string?> ShowStageAsync(string repoDir, int stage, string path, CancellationToken ct = default)
-    {
-        var (exitCode, stdout, _) = await RunGitAsync(repoDir, ["show", $":{stage}:{path}"], ct, trimOutput: false);
-        return exitCode == 0 ? stdout : null;
-    }
-
     public async Task AddPathAsync(string repoDir, string path, CancellationToken ct = default)
     {
         var (exitCode, _, stderr) = await RunGitAsync(repoDir, ["add", "--", path], ct);