feat(music): add optional Atlas Cloud provider

[binyangzhu000-sudo]

Summary

• add Atlas Cloud as an explicit optional text-to-music provider while keeping Mureka as the default
• persist Atlas prediction IDs so resumed jobs only poll and never repeat the generation POST
• add settings, credential probing, capability discovery, agent routing, Skill/help docs, and focused contract coverage

Validation

• npm run verify:affected (10/10 passed)
• npm run lint
• focused music, generation input, capability, keystore, key-probe, and Skill checks
• isolated TypeScript check for the Atlas provider/request validation modules
• real minimax/music-2.6 smoke: completed and downloaded a valid 2,512,705-byte MP3 (44.1 kHz stereo, 128 kbps)

Full npm run build could not be completed because this local sparse clone could not fetch the complete upstream source archive before the network timeout; the affected suite and modified core modules passed.