Expose avatar handle constraints in tool schemas

[chasehuh]

Summary\n\nFixes #13.\n\n- add a shared public avatar handle schema with live OpenAPI-aligned min/max length and pattern\n- apply it to avatars.create, the handle alias, Avatar MCP create schemas, and avatar-videos.create handle fields\n- add regression assertions that tool schema exports include the constraints\n\n## Validation\n\n- pnpm test -- test/cli.test.ts\n- pnpm run check\n- no-cost built CLI smoke for tools schema avatars.create and tools schema avatar-videos.create\n\nNo paid/provider calls, releases, deploys, or environment changes.\n