Fix YAML escaping for meeting note frontmatter

MeetingAssistant/MeetingNotes/MarkdownMeetingNoteStore.cs

@@ -1,4 +1,6 @@
 using Microsoft.Extensions.Options;
+using System.Globalization;
+using System.Text;
 using YamlDotNet.Core;
 using YamlDotNet.Serialization;
 
@@ -113,7 +115,24 @@ public sealed class MarkdownMeetingNoteStore : IMeetingNoteStore
 
     private static string EscapeQuoted(string value)
     {
-        return $"\"{value.Replace("\"", "\\\"", StringComparison.Ordinal)}\"";
+        var escaped = new StringBuilder(value.Length + 2);
+        escaped.Append('"');
+        foreach (var character in value)
+        {
+            escaped.Append(character switch
+            {
+                '\\' => "\\\\",
+                '"' => "\\\"",
+                '\r' => "\\r",
+                '\n' => "\\n",
+                '\t' => "\\t",
+                < ' ' => "\\x" + ((int)character).ToString("X2", CultureInfo.InvariantCulture),
+                _ => character.ToString()
+            });
+        }
+
+        escaped.Append('"');
+        return escaped.ToString();
     }
 
     private static string EscapeNullableDateTime(DateTimeOffset? value)
@@ -135,7 +154,7 @@ public sealed class MarkdownMeetingNoteStore : IMeetingNoteStore
             return "\"\"";
         }
 
-        if (value.Contains(':', StringComparison.Ordinal) || value.StartsWith("[", StringComparison.Ordinal))
+        if (RequiresQuotedScalar(value))
         {
             return EscapeQuoted(value);
         }
@@ -143,6 +162,49 @@ public sealed class MarkdownMeetingNoteStore : IMeetingNoteStore
         return value;
     }
 
+    private static bool RequiresQuotedScalar(string value)
+    {
+        if (char.IsWhiteSpace(value[0]) || char.IsWhiteSpace(value[^1]))
+        {
+            return true;
+        }
+
+        if (value.Contains(':', StringComparison.Ordinal) ||
+            value.Contains('\'', StringComparison.Ordinal) ||
+            value.Contains(" #", StringComparison.Ordinal) ||
+            value.Any(char.IsControl))
+        {
+            return true;
+        }
+
+        if (IsYamlIndicator(value[0]))
+        {
+            return true;
+        }
+
+        if (IsYamlCoreSchemaKeyword(value))
+        {
+            return true;
+        }
+
+        return DateTimeOffset.TryParse(value, out _) ||
+            double.TryParse(value, CultureInfo.InvariantCulture, out _);
+    }
+
+    private static bool IsYamlIndicator(char character)
+    {
+        return character is '-' or '?' or ':' or ',' or '[' or ']' or '{' or '}' or
+            '#' or '&' or '*' or '!' or '|' or '>' or '\'' or '"' or '%' or '@' or '`';
+    }
+
+    private static bool IsYamlCoreSchemaKeyword(string value)
+    {
+        return string.Equals(value, "true", StringComparison.OrdinalIgnoreCase) ||
+            string.Equals(value, "false", StringComparison.OrdinalIgnoreCase) ||
+            string.Equals(value, "null", StringComparison.OrdinalIgnoreCase) ||
+            value == "~";
+    }
+
     private sealed class MeetingNoteYaml
     {
         [YamlMember(Alias = "title")]