feat(merge): Stash-Option fuer uncommittete Aenderungen im Ziel-Tree

Ein dirty Ziel-Arbeitsverzeichnis gibt jetzt den eigenen Status
dirty_target zurueck statt blocked. Die UI bietet daraufhin einen
Retry an, der per git merge --autostash stasht, merged und die
Aenderungen danach wiederherstellt - opt-in pro Merge:

- Approve & Merge: Bestaetigungsdialog
- Merge-Modal: Button "Aenderungen stashen und mergen"
- MCP (merge_task/review_task) wirft weiterhin, kein autostash dort

src/ClaudeDo.Data/Git/GitService.cs

@@ -424,14 +424,22 @@ public sealed class GitService
         return File.Exists(Path.Combine(gitDir, "MERGE_HEAD"));
     }
 
+    /// <param name="autostash">Stash the target working tree's uncommitted changes before the
+    /// merge and restore them after it. Git owns the bookkeeping: on a conflict the stash stays
+    /// pending and is re-applied by `merge --continue`/`--abort`, so a conflicted merge left in
+    /// the tree doesn't lose it. A restore that itself conflicts leaves the stash in
+    /// `git stash list` rather than dropping it.</param>
     public async Task<(int ExitCode, string Stderr)> MergeNoFfAsync(
-        string repoDir, string sourceBranch, string message, CancellationToken ct = default)
+        string repoDir, string sourceBranch, string message, CancellationToken ct = default,
+        bool autostash = false)
     {
         // diff3 conflict style writes the merge base (|||||||) into conflict markers so the
         // in-app resolver can show a true three-way view. It only enriches conflicted hunks;
         // clean merges are unaffected.
-        var (exitCode, _, stderr) = await RunGitAsync(repoDir,
-            ["-c", "merge.conflictStyle=diff3", "merge", "--no-ff", "-m", message, sourceBranch], ct);
+        string[] args = autostash
+            ? ["-c", "merge.conflictStyle=diff3", "merge", "--no-ff", "--autostash", "-m", message, sourceBranch]
+            : ["-c", "merge.conflictStyle=diff3", "merge", "--no-ff", "-m", message, sourceBranch];
+        var (exitCode, _, stderr) = await RunGitAsync(repoDir, args, ct);
         return (exitCode, stderr);
     }