feat(ai): integrate Together AI adapter

[Photon101]

Summary

• replace the Together AI stub with an OpenAI-compatible chat completions implementation
• add current Together model IDs, dry-run behavior, usage-token mapping, and concise HTTP error messages
• cover the request shape, dry-run short-circuit, and error handling with focused Vitest tests

This implements one of the AI provider adapters requested in #6.

Validation

• git diff --check
• corepack pnpm exec vitest run packages/ai/together/src/index.test.ts
• corepack pnpm exec tsc -p packages/ai/together/tsconfig.json --noEmit

Note: the repo's full workspace install currently fails before reaching this adapter because packages/dns/azure depends on @sh1pt/core@workspace:*, while the workspace package is named @profullstack/sh1pt-core. I validated this package with root dev tooling and a local link to packages/core instead of changing unrelated workspace metadata.