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/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")
 )