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/recipient.go

@@ -0,0 +1,15 @@
+package packet
+
+// Recipient type represents a Intended Recipient Fingerprint subpacket
+// See https://datatracker.ietf.org/doc/html/draft-ietf-openpgp-crypto-refresh#name-intended-recipient-fingerpr
+type Recipient struct {
+	KeyVersion  int
+	Fingerprint []byte
+}
+
+func (r *Recipient) Serialize() []byte {
+	packet := make([]byte, len(r.Fingerprint)+1)
+	packet[0] = byte(r.KeyVersion)
+	copy(packet[1:], r.Fingerprint)
+	return packet
+}