feat(mcp): expose the conflict resolver's hunk API over MCP

get_merge_conflicts and resolve_conflict_hunk let an MCP caller resolve a
leaveConflictsInTree merge hunk-by-hunk (ours/theirs/base/literal text)
instead of grep-parsing diff3 markers itself. Reuses the existing
TaskMergeService/ConflictMarkerParser machinery (adds MergeSegment.StartLine
and a non-staging TaskMergeService.WriteConflictFileAsync) rather than a
second parser. A partially resolved file is written without git add so
continue_merge keeps refusing while any hunk still has markers.

tests/ClaudeDo.Data.Tests/ConflictMarkerParserTests.cs

@@ -68,6 +68,20 @@ public class ConflictMarkerParserTests
         Assert.Equal("Y\n", segments[1].Theirs);
     }
 
+    [Fact]
+    public void Segments_ExposeStartLine()
+    {
+        const string text =
+            "line1\nline2\n<<<<<<< HEAD\nours\n=======\ntheirs\n>>>>>>> branch\nline3\n";
+
+        var segments = ConflictMarkerParser.Parse(text);
+
+        Assert.Equal(3, segments.Count);
+        Assert.Equal(1, segments[0].StartLine);   // "line1\nline2\n"
+        Assert.Equal(3, segments[1].StartLine);   // the "<<<<<<<" line
+        Assert.Equal(8, segments[2].StartLine);   // "line3\n"
+    }
+
     [Fact]
     public void MultipleConflicts_AreEachCaptured()
     {