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/golang.org/x/crypto/ssh/mlkem.go

@@ -0,0 +1,168 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+import (
+	"crypto"
+	"crypto/mlkem"
+	"crypto/sha256"
+	"errors"
+	"fmt"
+	"io"
+
+	"golang.org/x/crypto/curve25519"
+)
+
+// mlkem768WithCurve25519sha256 implements the hybrid ML-KEM768 with
+// curve25519-sha256 key exchange method, as described by
+// draft-kampanakis-curdle-ssh-pq-ke-05 section 2.3.3.
+type mlkem768WithCurve25519sha256 struct{}
+
+func (kex *mlkem768WithCurve25519sha256) Client(c packetConn, rand io.Reader, magics *handshakeMagics) (*kexResult, error) {
+	var c25519kp curve25519KeyPair
+	if err := c25519kp.generate(rand); err != nil {
+		return nil, err
+	}
+
+	seed := make([]byte, mlkem.SeedSize)
+	if _, err := io.ReadFull(rand, seed); err != nil {
+		return nil, err
+	}
+
+	mlkemDk, err := mlkem.NewDecapsulationKey768(seed)
+	if err != nil {
+		return nil, err
+	}
+
+	hybridKey := append(mlkemDk.EncapsulationKey().Bytes(), c25519kp.pub[:]...)
+	if err := c.writePacket(Marshal(&kexECDHInitMsg{hybridKey})); err != nil {
+		return nil, err
+	}
+
+	packet, err := c.readPacket()
+	if err != nil {
+		return nil, err
+	}
+
+	var reply kexECDHReplyMsg
+	if err = Unmarshal(packet, &reply); err != nil {
+		return nil, err
+	}
+
+	if len(reply.EphemeralPubKey) != mlkem.CiphertextSize768+32 {
+		return nil, errors.New("ssh: peer's mlkem768x25519 public value has wrong length")
+	}
+
+	// Perform KEM decapsulate operation to obtain shared key from ML-KEM.
+	mlkem768Secret, err := mlkemDk.Decapsulate(reply.EphemeralPubKey[:mlkem.CiphertextSize768])
+	if err != nil {
+		return nil, err
+	}
+
+	// Complete Curve25519 ECDH to obtain its shared key.
+	c25519Secret, err := curve25519.X25519(c25519kp.priv[:], reply.EphemeralPubKey[mlkem.CiphertextSize768:])
+	if err != nil {
+		return nil, fmt.Errorf("ssh: peer's mlkem768x25519 public value is not valid: %w", err)
+	}
+	// Compute actual shared key.
+	h := sha256.New()
+	h.Write(mlkem768Secret)
+	h.Write(c25519Secret)
+	secret := h.Sum(nil)
+
+	h.Reset()
+	magics.write(h)
+	writeString(h, reply.HostKey)
+	writeString(h, hybridKey)
+	writeString(h, reply.EphemeralPubKey)
+
+	K := make([]byte, stringLength(len(secret)))
+	marshalString(K, secret)
+	h.Write(K)
+
+	return &kexResult{
+		H:         h.Sum(nil),
+		K:         K,
+		HostKey:   reply.HostKey,
+		Signature: reply.Signature,
+		Hash:      crypto.SHA256,
+	}, nil
+}
+
+func (kex *mlkem768WithCurve25519sha256) Server(c packetConn, rand io.Reader, magics *handshakeMagics, priv AlgorithmSigner, algo string) (*kexResult, error) {
+	packet, err := c.readPacket()
+	if err != nil {
+		return nil, err
+	}
+
+	var kexInit kexECDHInitMsg
+	if err = Unmarshal(packet, &kexInit); err != nil {
+		return nil, err
+	}
+
+	if len(kexInit.ClientPubKey) != mlkem.EncapsulationKeySize768+32 {
+		return nil, errors.New("ssh: peer's ML-KEM768/curve25519 public value has wrong length")
+	}
+
+	encapsulationKey, err := mlkem.NewEncapsulationKey768(kexInit.ClientPubKey[:mlkem.EncapsulationKeySize768])
+	if err != nil {
+		return nil, fmt.Errorf("ssh: peer's ML-KEM768 encapsulation key is not valid: %w", err)
+	}
+	// Perform KEM encapsulate operation to obtain ciphertext and shared key.
+	mlkem768Secret, mlkem768Ciphertext := encapsulationKey.Encapsulate()
+
+	// Perform server side of Curve25519 ECDH to obtain server public value and
+	// shared key.
+	var c25519kp curve25519KeyPair
+	if err := c25519kp.generate(rand); err != nil {
+		return nil, err
+	}
+	c25519Secret, err := curve25519.X25519(c25519kp.priv[:], kexInit.ClientPubKey[mlkem.EncapsulationKeySize768:])
+	if err != nil {
+		return nil, fmt.Errorf("ssh: peer's ML-KEM768/curve25519 public value is not valid: %w", err)
+	}
+	hybridKey := append(mlkem768Ciphertext, c25519kp.pub[:]...)
+
+	// Compute actual shared key.
+	h := sha256.New()
+	h.Write(mlkem768Secret)
+	h.Write(c25519Secret)
+	secret := h.Sum(nil)
+
+	hostKeyBytes := priv.PublicKey().Marshal()
+
+	h.Reset()
+	magics.write(h)
+	writeString(h, hostKeyBytes)
+	writeString(h, kexInit.ClientPubKey)
+	writeString(h, hybridKey)
+
+	K := make([]byte, stringLength(len(secret)))
+	marshalString(K, secret)
+	h.Write(K)
+
+	H := h.Sum(nil)
+
+	sig, err := signAndMarshal(priv, rand, H, algo)
+	if err != nil {
+		return nil, err
+	}
+
+	reply := kexECDHReplyMsg{
+		EphemeralPubKey: hybridKey,
+		HostKey:         hostKeyBytes,
+		Signature:       sig,
+	}
+	if err := c.writePacket(Marshal(&reply)); err != nil {
+		return nil, err
+	}
+	return &kexResult{
+		H:         H,
+		K:         K,
+		HostKey:   hostKeyBytes,
+		Signature: sig,
+		Hash:      crypto.SHA256,
+	}, nil
+}