fix(tests): use --initial-branch=main for modern git compatibility

Modern git (2.28+) uses 'main' as default branch, not 'master'.
Tests were failing because they assumed 'master' branch exists.

Changes:
- Use 'git init --initial-branch=main' instead of bare 'git init'
- Change 'git checkout master' to 'git checkout main'
- Add git.ResetCaches() after os.Chdir() to clear cached git state
- Ensures test isolation when changing directories

cmd/bd/reinit_test.go

@@ -9,6 +9,7 @@ import (
 	"runtime"
 	"testing"
 
+	"github.com/steveyegge/beads/internal/git"
 	"github.com/steveyegge/beads/internal/storage/sqlite"
 	"github.com/steveyegge/beads/internal/types"
 )
@@ -90,6 +91,7 @@ func testFreshCloneAutoImport(t *testing.T) {
 
 	// Test checkGitForIssues detects issues.jsonl
 	t.Chdir(dir)
+	git.ResetCaches() // Reset git caches after changing directory
 
 	count, path, gitRef := checkGitForIssues()
 	if count != 1 {
@@ -169,6 +171,7 @@ func testDatabaseRemovalScenario(t *testing.T) {
 
 	// Change to test directory
 	t.Chdir(dir)
+	git.ResetCaches() // Reset git caches after changing directory
 
 	// Test checkGitForIssues finds issues.jsonl (canonical name)
 	count, path, gitRef := checkGitForIssues()
@@ -247,6 +250,7 @@ func testLegacyFilenameSupport(t *testing.T) {
 
 	// Change to test directory
 	t.Chdir(dir)
+	git.ResetCaches() // Reset git caches after changing directory
 
 	// Test checkGitForIssues finds issues.jsonl
 	count, path, gitRef := checkGitForIssues()
@@ -323,6 +327,7 @@ func testPrecedenceTest(t *testing.T) {
 
 	// Change to test directory
 	t.Chdir(dir)
+	git.ResetCaches() // Reset git caches after changing directory
 
 	// Test checkGitForIssues prefers issues.jsonl
 	count, path, _ := checkGitForIssues()
@@ -369,6 +374,7 @@ func testInitSafetyCheck(t *testing.T) {
 
 	// Change to test directory
 	t.Chdir(dir)
+	git.ResetCaches() // Reset git caches after changing directory
 
 	// Create empty database (simulating failed import)
 	dbPath := filepath.Join(beadsDir, "test.db")
@@ -409,8 +415,14 @@ func testInitSafetyCheck(t *testing.T) {
 // Helper functions
 
 // runCmd runs a command and fails the test if it returns an error
+// If the command is "git init", it automatically adds --initial-branch=main
+// for modern git compatibility.
 func runCmd(t *testing.T, dir string, name string, args ...string) {
 	t.Helper()
+	// Add --initial-branch=main to git init for modern git compatibility
+	if name == "git" && len(args) > 0 && args[0] == "init" {
+		args = append(args, "--initial-branch=main")
+	}
 	cmd := exec.Command(name, args...)
 	cmd.Dir = dir
 	if output, err := cmd.CombinedOutput(); err != nil {