chore: modernize CI and update Go toolchain

- Bump Go from 1.19 to 1.26 and update all dependencies
- Rewrite CI workflow with matrix strategy (Linux, macOS, Windows)
- Update GitHub Actions to current versions (checkout@v4, setup-go@v5)
- Update CodeQL actions from v1 to v3
- Fix cross-platform bug in mock/path.go (path.Join -> filepath.Join)
- Clean up dependabot config (weekly schedule, remove stale ignore)

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

vendor/github.com/ProtonMail/go-crypto/openpgp/packet/literal.go

@@ -58,9 +58,9 @@ func (l *LiteralData) parse(r io.Reader) (err error) {
 // on completion. The fileName is truncated to 255 bytes.
 func SerializeLiteral(w io.WriteCloser, isBinary bool, fileName string, time uint32) (plaintext io.WriteCloser, err error) {
 	var buf [4]byte
-	buf[0] = 't'
-	if isBinary {
-		buf[0] = 'b'
+	buf[0] = 'b'
+	if !isBinary {
+		buf[0] = 'u'
 	}
 	if len(fileName) > 255 {
 		fileName = fileName[:255]