refactor(prompt): remove legacy rules fallback

internal/prompt/assembler.go

@@ -280,17 +280,12 @@ func osDescription() string {
 }
 
 func readRulesFile(dir string) (string, error) {
-	for _, name := range []string{"AGENTS.md", "CLAUDE.md"} {
-		path := filepath.Join(dir, name)
-		data, err := os.ReadFile(path)
-		if err == nil {
-			return string(data), nil
-		}
-		if !os.IsNotExist(err) {
-			return "", fmt.Errorf("read %s: %w", path, err)
-		}
+	path := filepath.Join(dir, "AGENTS.md")
+	data, err := os.ReadFile(path)
+	if err != nil && !os.IsNotExist(err) {
+		return "", fmt.Errorf("read %s: %w", path, err)
 	}
-	return "", nil
+	return string(data), nil
 }
 
 func detectOverrides(rulesContent string) map[string]bool {

internal/prompt/assembler_test.go

@@ -150,17 +150,12 @@ func TestAssembledBaselinePromptDoesNotAdvertiseMemoryTools(t *testing.T) {
 	}
 }
 
-func TestReadRulesFilePrefersAgentsThenClaude(t *testing.T) {
+func TestReadRulesFileAgentsOnly(t *testing.T) {
 	dir := t.TempDir()
 	{
 		got, err := readRulesFile(dir)
 		testassert.Require(t, err == nil && got == "", "empty readRulesFile = %q, %v", got, err)
 	}
-	testassert.NoError(t, os.WriteFile(filepath.Join(dir, "CLAUDE.md"), []byte("claude"), 0o600))
-	{
-		got, err := readRulesFile(dir)
-		testassert.Require(t, err == nil && got == "claude", "CLAUDE readRulesFile = %q, %v", got, err)
-	}
 	testassert.NoError(t, os.WriteFile(filepath.Join(dir, "AGENTS.md"), []byte("agents"), 0o600))
 	{
 		got, err := readRulesFile(dir)

internal/prompt/rules_file_guide.md

@@ -1,6 +1,6 @@
 ## Rules File Guide
 
-AGENTS.md is always present in your context (if AGENTS.md is missing, CLAUDE.md is used instead). It is where project-specific knowledge lives.
+AGENTS.md is where project-specific knowledge lives. Its contents are included in your context when the file is present.
 
 What belongs in it: