feat(social): implement X API posting

[thejaytang]

Summary

• replace the X social adapter fake success path with official X API v2 Post creation for mode=api
• post to /2/tweets with bearer user access tokens
• support text posts, replies, quote IDs, and pre-uploaded media IDs
• reject unsupported browser/raw media paths explicitly
• add social contract coverage plus focused mocked API tests for success, media validation, and X error responses

Context

Closes #257
References #6

X API docs used:

• https://docs.x.com/x-api/posts/manage-tweets/quickstart

Validation

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