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/scripts/src/templates/assets/usage.txt.tpl

@@ -5,3 +5,4 @@ Commands:
   dev [target] [-- args...]    Start with TypeScript and local-plugin source resolution
   build [args...] Run the project's installed tsdown
   config          Interactively edit project features
+  create <source> Add an external plugin dependency (pkg@version or github:owner/repo#ref) and mount it in cordis.yml

packages/sdk/scripts/tests/scripts.spec.ts

@@ -168,6 +168,7 @@ describe('Commander launcher arguments', () => {
     await expect(runDshSdkCommand(['unknown'], context)).resolves.toBe(1)
     await expect(runDshSdkCommand([], context)).resolves.toBe(0)
     expect(context.readStdout()).toContain('Usage: dsh-sdk')
+    expect(context.readStdout()).toContain('create <source>')
 
     const defaults = commandContext(root)
     await writeFile(join(root, 'main.mjs'), 'export function main() { return "ok" }\n')