Switch remote deploy to vendored source builds

Move remote deployment to a vendored source bundle built on the target host via Docker so redeploys no longer require local cross-compilation or host Go installation.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

vendor/github.com/labstack/gommon/log/white.go

@@ -0,0 +1,12 @@
+// +build appengine
+
+package log
+
+import (
+	"io"
+	"os"
+)
+
+func output() io.Writer {
+	return os.Stdout
+}