feat: add text-to-motion API support

[yujiaxu-ops]

Summary

• add a v1 text-to-motion client endpoint and create/get/list/wait/delete CLI resource
• expose --prompt, --mode prime|swift (Prime by default), and 2–10s --duration validation in 0.5s increments
• preserve result.motion_url responses and use motion_format when naming downloaded FBX/BVH clips
• document the command, credit costs, and standalone-clip semantics in the README and bundled skill

Examples

meshy text-to-motion create --prompt "a character waving" --duration 3
meshy text-to-motion create --prompt "a quick dodge left" --duration 2.5 --mode swift -o motion.bvh

Tests

• pnpm typecheck
• pnpm test (363 passed)
• pnpm build
• verified meshy text-to-motion create --help and preflight rejection of invalid --duration 2.3