fix(prompts): make template token substitution order-independent

RenderTemplate replaced tokens one key at a time over a StringBuilder, so a
token appearing inside an already-substituted value got substituted again on a
later pass. The prompt briefs only escaped this because their callers happen to
pass "tasks" last -- reordering the dictionary or adding a fourth token would
have started rewriting user-authored task descriptions, which after the enhance
phase carry file paths and config snippets.

Single-pass regex over the template instead; unknown tokens still pass through.

src/ClaudeDo.Data/PromptFiles.cs

@@ -1,6 +1,7 @@
 using System.Security.Cryptography;
 using System.Text;
 using System.Text.Json;
+using System.Text.RegularExpressions;
 
 namespace ClaudeDo.Data;
 
@@ -193,14 +194,16 @@ public static class PromptFiles
     public static string Render(PromptKind kind, IReadOnlyDictionary<string, string> values)
         => RenderTemplate(ReadOrDefault(kind), values);
 
-    /// <summary>Replace only the given {name} tokens; any other braces pass through untouched.</summary>
+    private static readonly Regex TokenPattern = new(@"\{(\w+)\}", RegexOptions.Compiled);
+
+    /// <summary>Replace only the given {name} tokens; any other braces pass through untouched.
+    /// Single pass over the template, so a token appearing inside a substituted VALUE is never
+    /// re-substituted. That matters because the values are user-authored task titles and
+    /// descriptions: a sharpened description mentioning a literal {repo} must survive verbatim,
+    /// and it must not depend on the caller happening to order its dictionary correctly.</summary>
     public static string RenderTemplate(string template, IReadOnlyDictionary<string, string> values)
-    {
-        var sb = new StringBuilder(template);
-        foreach (var (key, val) in values)
-            sb.Replace("{" + key + "}", val);
-        return sb.ToString();
-    }
+        => TokenPattern.Replace(template, m =>
+            values.TryGetValue(m.Groups[1].Value, out var val) ? val : m.Value);
 
     public static string DefaultFor(PromptKind kind) => kind switch
     {