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/version_test.go

@@ -0,0 +1,78 @@
+package main
+
+import (
+	"bytes"
+	"encoding/json"
+	"os"
+	"strings"
+	"testing"
+)
+
+func TestVersionCommand(t *testing.T) {
+	// Save original stdout
+	oldStdout := os.Stdout
+	defer func() { os.Stdout = oldStdout }()
+
+	t.Run("plain text version output", func(t *testing.T) {
+		// Create a pipe to capture output
+		r, w, err := os.Pipe()
+		if err != nil {
+			t.Fatalf("Failed to create pipe: %v", err)
+		}
+		os.Stdout = w
+		jsonOutput = false
+
+		// Run version command
+		versionCmd.Run(versionCmd, []string{})
+
+		// Close writer and read output
+		w.Close()
+		var buf bytes.Buffer
+		buf.ReadFrom(r)
+		output := buf.String()
+
+		// Verify output contains version info
+		if !strings.Contains(output, "bd version") {
+			t.Errorf("Expected output to contain 'bd version', got: %s", output)
+		}
+		if !strings.Contains(output, Version) {
+			t.Errorf("Expected output to contain version %s, got: %s", Version, output)
+		}
+	})
+
+	t.Run("json version output", func(t *testing.T) {
+		// Create a pipe to capture output
+		r, w, err := os.Pipe()
+		if err != nil {
+			t.Fatalf("Failed to create pipe: %v", err)
+		}
+		os.Stdout = w
+		jsonOutput = true
+
+		// Run version command
+		versionCmd.Run(versionCmd, []string{})
+
+		// Close writer and read output
+		w.Close()
+		var buf bytes.Buffer
+		buf.ReadFrom(r)
+		output := buf.String()
+
+		// Parse JSON output
+		var result map[string]string
+		if err := json.Unmarshal([]byte(output), &result); err != nil {
+			t.Fatalf("Failed to parse JSON output: %v", err)
+		}
+
+		// Verify JSON contains version and build
+		if result["version"] != Version {
+			t.Errorf("Expected version %s, got %s", Version, result["version"])
+		}
+		if result["build"] == "" {
+			t.Error("Expected build field to be non-empty")
+		}
+	})
+
+	// Restore default
+	jsonOutput = false
+}