feat(assets): wire builder into dispatch, shim, smoke, docs

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

cmd/crucible-assets/main.go

@@ -0,0 +1,11 @@
+// Command crucible-assets is the standalone assets builder (equivalent to
+// `crucible assets`). A single-token binary keeps consumer wrapper scripts simple.
+package main
+
+import (
+	"os"
+
+	"git.westgate.pw/ShadowsOverWestgate/sow-tools/internal/dispatch"
+)
+
+func main() { os.Exit(dispatch.RunBuilder("assets", os.Args[1:])) }