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/canonical_text.go

@@ -30,8 +30,12 @@ func writeCanonical(cw io.Writer, buf []byte, s *int) (int, error) {
 			if c == '\r' {
 				*s = 1
 			} else if c == '\n' {
-				cw.Write(buf[start:i])
-				cw.Write(newline)
+				if _, err := cw.Write(buf[start:i]); err != nil {
+					return 0, err
+				}
+				if _, err := cw.Write(newline); err != nil {
+					return 0, err
+				}
 				start = i + 1
 			}
 		case 1:
@@ -39,7 +43,9 @@ func writeCanonical(cw io.Writer, buf []byte, s *int) (int, error) {
 		}
 	}
 
-	cw.Write(buf[start:])
+	if _, err := cw.Write(buf[start:]); err != nil {
+		return 0, err
+	}
 	return len(buf), nil
 }