fix(sdk): list create <source> and headless flags in usage help

dsh-sdk's usage template predates the create command and create-sdk's
predates --config/--config-json/--json, so --help hid both surfaces the
README already documents. Pin each with a help-output assertion.

packages/sdk/create-sdk/src/templates/assets/usage.txt.tpl

@@ -9,3 +9,6 @@ Options:
   --interface <acp|stdio|embed>
   --pm <npm|pnpm|yarn>
   --install / --no-install
+  --config <path>
+  --config-json <json>
+  --json

packages/sdk/create-sdk/tests/create.spec.ts

@@ -477,6 +477,7 @@ describe('create command composition', () => {
     context.stdout.isTTY = false
     await expect(createProject(['--help'], context)).resolves.toBeUndefined()
     expect(context.readStdout()).toContain('Usage: create-sdk')
+    expect(context.readStdout()).toContain('--config-json <json>')
     expect(context.readStdout()).not.toContain('--link-workspace')
     await expect(createProject(argv('agent', false), context)).rejects.toThrow('interactive TTY')
     context.stdin.isTTY = true