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>

examples/library-usage/main_test.go

@@ -152,12 +152,7 @@ func TestFindDatabasePath(t *testing.T) {
 	f.Close()
 
 	// Change to temp directory
-	originalWd, _ := os.Getwd()
-	defer os.Chdir(originalWd)
-
-	if err := os.Chdir(tmpDir); err != nil {
-		t.Fatalf("Failed to chdir: %v", err)
-	}
+	t.Chdir(tmpDir)
 
 	// Test FindDatabasePath
 	found := beads.FindDatabasePath()