Make agent file writes append by default

MeetingAssistant.Tests/MeetingSummaryToolTests.cs

@@ -503,12 +503,42 @@ public sealed class MeetingSummaryToolTests
             """);
         var tools = new MeetingSummaryTools(artifacts);
 
+        await tools.WriteContext("appended");
         await tools.WriteContext("inserted", insert: 2);
         await tools.WriteContext("replacement", from: 1, to: 1);
 
         var context = await File.ReadAllTextAsync(artifacts.AssistantContextPath);
         Assert.Contains("state: summarizing", context);
-        Assert.Contains("replacement\ninserted\nline two", context);
+        Assert.Contains("replacement\ninserted\nline two\nappended", context);
+    }
+
+    [Fact]
+    public async Task ToolsReplaceAssistantContextBodyOnlyWhenRequested()
+    {
+        var root = Path.Combine(Path.GetTempPath(), "meeting-assistant-tests", Guid.NewGuid().ToString("N"));
+        var artifacts = new MeetingSessionArtifacts(
+            MeetingNotePath: Path.Combine(root, "Meetings", "Notes", "meeting.md"),
+            TranscriptPath: Path.Combine(root, "Meetings", "Transcripts", "transcript.md"),
+            AssistantContextPath: Path.Combine(root, "Meetings", "Assistant Context", "context.md"),
+            SummaryPath: Path.Combine(root, "Meetings", "Summaries", "summary.md"));
+        Directory.CreateDirectory(Path.GetDirectoryName(artifacts.AssistantContextPath)!);
+        await File.WriteAllTextAsync(
+            artifacts.AssistantContextPath,
+            """
+            ---
+            state: summarizing
+            ---
+
+            line one
+            """);
+        var tools = new MeetingSummaryTools(artifacts);
+
+        await tools.WriteContext("replacement", replace_file: true);
+
+        var context = await File.ReadAllTextAsync(artifacts.AssistantContextPath);
+        Assert.Contains("state: summarizing", context);
+        Assert.Contains("replacement", context);
+        Assert.DoesNotContain("line one", context);
     }
 
     [Fact]
@@ -561,7 +591,8 @@ public sealed class MeetingSummaryToolTests
             # Assistant Context
 
             ## Live Context
-            """);
+            """,
+            replace_file: true);
 
         var context = await File.ReadAllTextAsync(artifacts.AssistantContextPath);
         Assert.Contains("state: summarizing", context);