fix(bugseti): add gh CLI availability check with helpful error

Adds a startup check that verifies gh is in PATH and authenticated
before initializing services. Provides clear install/auth instructions
on failure instead of cryptic exec errors at runtime.

Closes #61

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cmd/bugseti/main.go

@@ -39,6 +39,11 @@ func main() {
 		log.Printf("Warning: Could not load config: %v", err)
 	}
 
+	// Check gh CLI availability
+	if err := bugseti.CheckGHCLI(); err != nil {
+		log.Fatalf("GitHub CLI check failed: %v", err)
+	}
+
 	// Initialize core services
 	notifyService := bugseti.NewNotifyService(configService)
 	statsService := bugseti.NewStatsService(configService)

internal/bugseti/ghcheck.go

@@ -0,0 +1,30 @@
+package bugseti
+
+import (
+	"fmt"
+	"os/exec"
+)
+
+// CheckGHCLI verifies that the gh CLI is installed and authenticated.
+// Returns nil if gh is available and logged in, or an error with
+// actionable instructions for the user.
+func CheckGHCLI() error {
+	// Check if gh is in PATH
+	if _, err := exec.LookPath("gh"); err != nil {
+		return fmt.Errorf("gh CLI not found in PATH: %w\n\n"+
+			"BugSETI requires the GitHub CLI (gh) to fetch issues and submit PRs.\n"+
+			"Install it from: https://cli.github.com\n\n"+
+			"  macOS:   brew install gh\n"+
+			"  Linux:   https://github.com/cli/cli/blob/trunk/docs/install_linux.md\n"+
+			"  Windows: winget install --id GitHub.cli", err)
+	}
+
+	// Check if gh is authenticated
+	cmd := exec.Command("gh", "auth", "status")
+	if out, err := cmd.CombinedOutput(); err != nil {
+		return fmt.Errorf("gh CLI is not authenticated: %w\n%s\n\n"+
+			"Run 'gh auth login' to authenticate with GitHub.", err, out)
+	}
+
+	return nil
+}

internal/bugseti/ghcheck_test.go

@@ -0,0 +1,37 @@
+package bugseti
+
+import (
+	"os/exec"
+	"strings"
+	"testing"
+)
+
+func TestCheckGHCLI_Good(t *testing.T) {
+	// Only run if gh is actually available (CI-friendly skip)
+	if _, err := exec.LookPath("gh"); err != nil {
+		t.Skip("gh CLI not installed, skipping")
+	}
+
+	err := CheckGHCLI()
+	// We can't guarantee auth status in all environments,
+	// but if gh is present the function should at least not panic.
+	if err != nil {
+		t.Logf("CheckGHCLI returned error (may be expected if not authenticated): %v", err)
+	}
+}
+
+func TestCheckGHCLI_Bad_MissingBinary(t *testing.T) {
+	// Save and clear PATH to simulate missing gh
+	t.Setenv("PATH", t.TempDir())
+
+	err := CheckGHCLI()
+	if err == nil {
+		t.Fatal("expected error when gh is not in PATH")
+	}
+	if !strings.Contains(err.Error(), "gh CLI not found") {
+		t.Errorf("error should mention 'gh CLI not found', got: %v", err)
+	}
+	if !strings.Contains(err.Error(), "https://cli.github.com") {
+		t.Errorf("error should include install URL, got: %v", err)
+	}
+}