feat(social): implement Threads publishing

[thejaytang]

Summary

• replace the Threads social adapter stub with Meta Threads API publishing calls
• create media containers through /{threads-user-id}/threads and publish them through /{threads-user-id}/threads_publish
• support text, public image URL, and public video URL single-post payloads
• add social contract coverage plus focused mocked API tests for success, media validation, and Meta error responses

Context

Closes #255
References #6

Meta Threads docs used:

• https://developers.facebook.com/docs/threads/posts/

Validation

• corepack pnpm --filter @profullstack/sh1pt-social-threads typecheck
• corepack pnpm vitest run packages/social/threads/src/index.test.ts
• corepack pnpm --filter @profullstack/sh1pt-social-threads build