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/ssh_gss.go

@@ -106,6 +106,13 @@ func parseGSSAPIPayload(payload []byte) (*userAuthRequestGSSAPI, error) {
 	if !ok {
 		return nil, errors.New("parse uint32 failed")
 	}
+	// Each ASN.1 encoded OID must have a minimum
+	// of 2 bytes; 64 maximum mechanisms is an
+	// arbitrary, but reasonable ceiling.
+	const maxMechs = 64
+	if n > maxMechs || int(n)*2 > len(rest) {
+		return nil, errors.New("invalid mechanism count")
+	}
 	s := &userAuthRequestGSSAPI{
 		N:    n,
 		OIDS: make([]asn1.ObjectIdentifier, n),
@@ -122,7 +129,6 @@ func parseGSSAPIPayload(payload []byte) (*userAuthRequestGSSAPI, error) {
 		if rest, err = asn1.Unmarshal(desiredMech, &s.OIDS[i]); err != nil {
 			return nil, err
 		}
-
 	}
 	return s, nil
 }