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/sys/unix/syscall_dragonfly.go

@@ -246,6 +246,18 @@ func Sendfile(outfd int, infd int, offset *int64, count int) (written int, err e
 	return sendfile(outfd, infd, offset, count)
 }
 
+func Dup3(oldfd, newfd, flags int) error {
+	if oldfd == newfd || flags&^O_CLOEXEC != 0 {
+		return EINVAL
+	}
+	how := F_DUP2FD
+	if flags&O_CLOEXEC != 0 {
+		how = F_DUP2FD_CLOEXEC
+	}
+	_, err := fcntl(oldfd, how, newfd)
+	return err
+}
+
 /*
  * Exposed directly
  */