Add optional Xquik target adapter

[kriptoburak]

Summary

• Add an opt-in Xquik target adapter for posting mirrored Bluesky posts to X.
• Pass source public media URLs through the adapter interface while keeping existing X and Instagram adapters on local downloaded files.
• Document the Xquik environment variables and keep all targets disabled unless their credentials are present.

Validation

• python3.11 -m compileall adapters core utils config.py main.py
• XquikAdapter request mock with a Python 3.11 temp venv
• git diff --check