fix: build api CLI as ESM to support trpc-cli

trpc-cli requires ESM with top-level await, which can't be loaded via
CommonJS require(). Bundle api.ts separately using esbuild as ESM (.mjs)
and use native import() via Function constructor to bypass TypeScript's
conversion to require().

Also adds `make mux` target for convenient CLI invocation.

Author: ThomasK33

Makefile

@@ -109,7 +109,7 @@ rebuild-native: node_modules/.installed ## Rebuild native modules (node-pty) for
 
 # Run compiled CLI with trailing arguments (builds only if missing)
 mux: ## Run the compiled mux CLI (e.g., make mux server --port 3000)
-	@test -f dist/cli/index.js || $(MAKE) build-main
+	@test -f dist/cli/index.js -a -f dist/cli/api.mjs || $(MAKE) build-main
 	@node dist/cli/index.js $(filter-out $@,$(MAKECMDGOALS))
 
 # Catch unknown targets passed to mux (prevents "No rule to make target" errors)