test: replace manual os.Chdir with t.Chdir in tests (#457)

Replaces manual working directory save/restore patterns
with Go's built-in `t.Chdir()` helper across 23 test files.

The manual pattern involved calling `os.Getwd()` to save
the original directory, using `defer os.Chdir(origWd)` for
restoration, and manually handling errors during directory
changes. This boilerplate has been replaced with single
`t.Chdir(path)` calls that handle cleanup automatically.

The `t.Chdir()` method automatically restores the working
directory when the test completes, eliminating the need for
manual defer statements and error handling.

Total:
~75 instances replaced (assuming Claude's math is right)

Co-authored-by: Claude <noreply@anthropic.com>

cmd/bd/nodb_test.go

@@ -141,15 +141,11 @@ func TestDetectPrefix(t *testing.T) {
 
 	t.Run("empty database defaults to dir name", func(t *testing.T) {
 		// Change to temp dir so we can control directory name
-		origWd, _ := os.Getwd()
 		namedDir := filepath.Join(tempDir, "myproject")
 		if err := os.MkdirAll(namedDir, 0o755); err != nil {
 			t.Fatalf("Failed to create named dir: %v", err)
 		}
-		if err := os.Chdir(namedDir); err != nil {
-			t.Fatalf("Failed to chdir: %v", err)
-		}
-		defer func() { _ = os.Chdir(origWd) }()
+		t.Chdir(namedDir)
 
 		memStore := memory.New(filepath.Join(beadsDir, "issues.jsonl"))
 		prefix, err := detectPrefix(beadsDir, memStore)