Implement X API social posting adapter

[thejaytang]

Scope

Implement the existing social-x package for official X API v2 post creation in api mode.

Planned changes

• POST to /2/tweets with OAuth bearer user access token
• Support text posts, replies, quote IDs, and pre-uploaded media IDs
• Reject browser mode and raw media upload paths explicitly instead of returning a fake post id
• Add focused contract and API-shape tests

Umbrella bounty context: #6