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/cloudflare/circl/sign/ed25519/point.go

@@ -164,7 +164,7 @@ func (P *pointR1) isEqual(Q *pointR1) bool {
 	fp.Mul(r, r, &P.z)
 	fp.Sub(l, l, r)
 	b = b && fp.IsZero(l)
-	return b
+	return b && !fp.IsZero(&P.z) && !fp.IsZero(&Q.z)
 }
 
 func (P *pointR3) neg() {

vendor/github.com/cloudflare/circl/sign/ed448/ed448.go

@@ -206,7 +206,7 @@ func newKeyFromSeed(privateKey, seed []byte) {
 
 func signAll(signature []byte, privateKey PrivateKey, message, ctx []byte, preHash bool) {
 	if len(ctx) > ContextMaxSize {
-		panic(fmt.Errorf("ed448: bad context length: " + strconv.Itoa(len(ctx))))
+		panic(fmt.Errorf("ed448: bad context length: %v", len(ctx)))
 	}
 
 	H := sha3.NewShake256()

vendor/github.com/cloudflare/circl/sign/sign.go

@@ -38,6 +38,12 @@ type PrivateKey interface {
 	encoding.BinaryMarshaler
 }
 
+// A private key that retains the seed with which it was generated.
+type Seeded interface {
+	// returns the seed if retained, otherwise nil
+	Seed() []byte
+}
+
 // A Scheme represents a specific instance of a signature scheme.
 type Scheme interface {
 	// Name of the scheme.
@@ -107,4 +113,7 @@ var (
 	// ErrContextNotSupported is the error used if a context is not
 	// supported.
 	ErrContextNotSupported = errors.New("context not supported")
+
+	// ErrContextTooLong is the error used if the context string is too long.
+	ErrContextTooLong = errors.New("context string too long")
 )