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/internal/ecc/curve_info.go

@@ -4,11 +4,14 @@ package ecc
 import (
 	"bytes"
 	"crypto/elliptic"
+
 	"github.com/ProtonMail/go-crypto/bitcurves"
 	"github.com/ProtonMail/go-crypto/brainpool"
 	"github.com/ProtonMail/go-crypto/openpgp/internal/encoding"
 )
 
+const Curve25519GenName = "Curve25519"
+
 type CurveInfo struct {
 	GenName string
 	Oid     *encoding.OID
@@ -42,19 +45,19 @@ var Curves = []CurveInfo{
 	},
 	{
 		// Curve25519
-		GenName: "Curve25519",
+		GenName: Curve25519GenName,
 		Oid:     encoding.NewOID([]byte{0x2B, 0x06, 0x01, 0x04, 0x01, 0x97, 0x55, 0x01, 0x05, 0x01}),
 		Curve:   NewCurve25519(),
 	},
 	{
-		// X448
+		// x448
 		GenName: "Curve448",
 		Oid:     encoding.NewOID([]byte{0x2B, 0x65, 0x6F}),
 		Curve:   NewX448(),
 	},
 	{
 		// Ed25519
-		GenName: "Curve25519",
+		GenName: Curve25519GenName,
 		Oid:     encoding.NewOID([]byte{0x2B, 0x06, 0x01, 0x04, 0x01, 0xDA, 0x47, 0x0F, 0x01}),
 		Curve:   NewEd25519(),
 	},

vendor/github.com/ProtonMail/go-crypto/openpgp/internal/ecc/ed25519.go

@@ -2,6 +2,7 @@
 package ecc
 
 import (
+	"bytes"
 	"crypto/subtle"
 	"io"
 
@@ -90,7 +91,14 @@ func (c *ed25519) GenerateEdDSA(rand io.Reader) (pub, priv []byte, err error) {
 }
 
 func getEd25519Sk(publicKey, privateKey []byte) ed25519lib.PrivateKey {
-	return append(privateKey, publicKey...)
+	privateKeyCap, privateKeyLen, publicKeyLen := cap(privateKey), len(privateKey), len(publicKey)
+
+	if privateKeyCap >= privateKeyLen+publicKeyLen &&
+		bytes.Equal(privateKey[privateKeyLen:privateKeyLen+publicKeyLen], publicKey) {
+		return privateKey[:privateKeyLen+publicKeyLen]
+	}
+
+	return append(privateKey[:privateKeyLen:privateKeyLen], publicKey...)
 }
 
 func (c *ed25519) Sign(publicKey, privateKey, message []byte) (sig []byte, err error) {

vendor/github.com/ProtonMail/go-crypto/openpgp/internal/ecc/ed448.go

@@ -2,6 +2,7 @@
 package ecc
 
 import (
+	"bytes"
 	"crypto/subtle"
 	"io"
 
@@ -84,7 +85,14 @@ func (c *ed448) GenerateEdDSA(rand io.Reader) (pub, priv []byte, err error) {
 }
 
 func getEd448Sk(publicKey, privateKey []byte) ed448lib.PrivateKey {
-	return append(privateKey, publicKey...)
+	privateKeyCap, privateKeyLen, publicKeyLen := cap(privateKey), len(privateKey), len(publicKey)
+
+	if privateKeyCap >= privateKeyLen+publicKeyLen &&
+		bytes.Equal(privateKey[privateKeyLen:privateKeyLen+publicKeyLen], publicKey) {
+		return privateKey[:privateKeyLen+publicKeyLen]
+	}
+
+	return append(privateKey[:privateKeyLen:privateKeyLen], publicKey...)
 }
 
 func (c *ed448) Sign(publicKey, privateKey, message []byte) (sig []byte, err error) {