fix: remaining staticcheck QF1012 WriteString(Sprintf) violations

internal/core/edit.go

@@ -194,7 +194,7 @@ func generateDiff(path, old, new string, changeIdx int) string {
 		if i >= lineNum-1 && i < lineNum-1+countNewlines(old[changeIdx:])+1 {
 			prefix = "-"
 		}
-		diff.WriteString(fmt.Sprintf("%s %s\n", prefix, oldLines[i]))
+		fmt.Fprintf(&diff, "%s %s\n", prefix, oldLines[i])
 	}
 
 	return diff.String()

internal/core/ls.go

@@ -82,7 +82,7 @@ func executeLs(ctx context.Context, call fantasy.ToolCall, workDir string) (fant
 	count := 0
 	for _, entry := range entries {
 		if count >= limit {
-			result.WriteString(fmt.Sprintf("\n[truncated: showing %d of %d entries]", limit, len(entries)))
+			fmt.Fprintf(&result, "\n[truncated: showing %d of %d entries]", limit, len(entries))
 			break
 		}
 		name := entry.Name()

internal/core/read.go

@@ -100,7 +100,7 @@ func executeRead(ctx context.Context, call fantasy.ToolCall, workDir string) (fa
 	var result strings.Builder
 	for i, line := range lines {
 		lineNum := offset + i + 1
-		result.WriteString(fmt.Sprintf("%d: %s\n", lineNum, line))
+		fmt.Fprintf(&result, "%d: %s\n", lineNum, line)
 	}
 
 	output := result.String()

internal/skills/prompt_builder.go

@@ -65,7 +65,7 @@ func (pb *PromptBuilder) Build() string {
 			buf.WriteString("\n\n")
 		}
 		if s.name != "" {
-			buf.WriteString(fmt.Sprintf("# %s\n\n", s.name))
+			fmt.Fprintf(&buf, "# %s\n\n", s.name)
 		}
 		buf.WriteString(s.content)
 	}

internal/skills/skills.go

@@ -216,11 +216,11 @@ func FormatForPrompt(skills []*Skill) string {
 
 	for _, s := range skills {
 		buf.WriteString("  <skill>\n")
-		buf.WriteString(fmt.Sprintf("    <name>%s</name>\n", s.Name))
+		fmt.Fprintf(&buf, "    <name>%s</name>\n", s.Name)
 		if s.Description != "" {
-			buf.WriteString(fmt.Sprintf("    <description>%s</description>\n", s.Description))
+			fmt.Fprintf(&buf, "    <description>%s</description>\n", s.Description)
 		}
-		buf.WriteString(fmt.Sprintf("    <location>file://%s</location>\n", s.Path))
+		fmt.Fprintf(&buf, "    <location>file://%s</location>\n", s.Path)
 		buf.WriteString("  </skill>\n")
 	}