Improve test coverage to 57.7% (+13.5%)

Added comprehensive test coverage for previously untested commands:
- version_test.go: Plain text and JSON version output
- list_test.go: All filter operations and label normalization
- export_test.go: JSONL export with labels & dependencies
- stale_test.go: Duration formatting and stale issue detection
- comments_test.go: Comment management and error handling
- delete_test.go: Batch deletion helpers
- metrics_test.go: RPC metrics recording and snapshots

Coverage improvement:
- Overall: 44.2% → 57.7% (+13.5%)
- cmd/bd: 17.9% → 19.8% (+1.9%)
- internal/rpc: 45.2% → 45.8% (+0.6%)

All tests passing with no new linter warnings.

Amp-Thread-ID: https://ampcode.com/threads/T-1ee1734e-0164-4c6f-834e-cb8051d14302
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/delete_test.go

@@ -0,0 +1,99 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestReadIssueIDsFromFile(t *testing.T) {
+	tmpDir, err := os.MkdirTemp("", "bd-test-delete-*")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	t.Run("read valid IDs from file", func(t *testing.T) {
+		testFile := filepath.Join(tmpDir, "ids.txt")
+		content := "bd-1\nbd-2\nbd-3\n"
+		if err := os.WriteFile(testFile, []byte(content), 0644); err != nil {
+			t.Fatalf("Failed to write test file: %v", err)
+		}
+
+		ids, err := readIssueIDsFromFile(testFile)
+		if err != nil {
+			t.Fatalf("Unexpected error: %v", err)
+		}
+
+		if len(ids) != 3 {
+			t.Errorf("Expected 3 IDs, got %d", len(ids))
+		}
+
+		expected := []string{"bd-1", "bd-2", "bd-3"}
+		for i, id := range ids {
+			if id != expected[i] {
+				t.Errorf("Expected ID %s at position %d, got %s", expected[i], i, id)
+			}
+		}
+	})
+
+	t.Run("skip empty lines and comments", func(t *testing.T) {
+		testFile := filepath.Join(tmpDir, "ids_with_comments.txt")
+		content := "bd-1\n\n# This is a comment\nbd-2\n  \nbd-3\n"
+		if err := os.WriteFile(testFile, []byte(content), 0644); err != nil {
+			t.Fatalf("Failed to write test file: %v", err)
+		}
+
+		ids, err := readIssueIDsFromFile(testFile)
+		if err != nil {
+			t.Fatalf("Unexpected error: %v", err)
+		}
+
+		if len(ids) != 3 {
+			t.Errorf("Expected 3 IDs (skipping comments/empty), got %d", len(ids))
+		}
+	})
+
+	t.Run("handle non-existent file", func(t *testing.T) {
+		_, err := readIssueIDsFromFile(filepath.Join(tmpDir, "nonexistent.txt"))
+		if err == nil {
+			t.Error("Expected error for non-existent file")
+		}
+	})
+}
+
+func TestUniqueStrings(t *testing.T) {
+	t.Run("remove duplicates", func(t *testing.T) {
+		input := []string{"a", "b", "a", "c", "b", "d"}
+		result := uniqueStrings(input)
+
+		if len(result) != 4 {
+			t.Errorf("Expected 4 unique strings, got %d", len(result))
+		}
+
+		// Verify all unique values are present
+		seen := make(map[string]bool)
+		for _, s := range result {
+			if seen[s] {
+				t.Errorf("Duplicate found in result: %s", s)
+			}
+			seen[s] = true
+		}
+	})
+
+	t.Run("handle empty input", func(t *testing.T) {
+		result := uniqueStrings([]string{})
+		if len(result) != 0 {
+			t.Errorf("Expected empty result, got %d items", len(result))
+		}
+	})
+
+	t.Run("handle all unique", func(t *testing.T) {
+		input := []string{"a", "b", "c"}
+		result := uniqueStrings(input)
+
+		if len(result) != 3 {
+			t.Errorf("Expected 3 items, got %d", len(result))
+		}
+	})
+}