feat: add cmd/tapir entrypoint stub

Minimal main that identifies the binary (gives the CI smoke test something to
grep). HTTP server, watcher, adapter wiring, and config come with the build.

cmd/tapir/main.go

@@ -0,0 +1,10 @@
+// Command tapir is the service entrypoint. Scaffold: it identifies itself so
+// the CI smoke test has something to grep for, and exits. Wiring the HTTP
+// server, watcher, adapters, and config is part of the build.
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("tapir: scaffold — not yet implemented")
+}