feat(git): add conflict-stage blob reads and single-path staging

src/ClaudeDo.Data/Git/GitService.cs

@@ -238,6 +238,24 @@ 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", $":{stage}:{path}"], ct, trimOutput: false);
+        return exitCode == 0 ? stdout : null;
+    }
+
+    public async Task AddPathAsync(string repoDir, string path, CancellationToken ct = default)
+    {
+        var (exitCode, _, stderr) = await RunGitAsync(repoDir, ["add", "--", path], ct);
+        if (exitCode != 0)
+            throw new InvalidOperationException($"git add '{path}' failed (exit {exitCode}): {stderr}");
+    }
+
     /// <summary>
     /// Non-destructive mergeability probe via `git merge-tree --write-tree`. Writes only
     /// loose objects — the working tree, index, and refs are left untouched.
@@ -289,7 +307,7 @@ public sealed class GitService
     }
 
     private static async Task<(int ExitCode, string Stdout, string Stderr)> RunGitAsync(
-        string workDir, IEnumerable<string> args, CancellationToken ct, string? stdinData = null)
+        string workDir, IEnumerable<string> args, CancellationToken ct, string? stdinData = null, bool trimOutput = true)
     {
         var psi = new ProcessStartInfo
         {
@@ -338,6 +356,6 @@ public sealed class GitService
 
         ct.ThrowIfCancellationRequested();
 
-        return (proc.ExitCode, stdout.TrimEnd(), stderr.TrimEnd());
+        return (proc.ExitCode, trimOutput ? stdout.TrimEnd() : stdout, stderr.TrimEnd());
     }
 }