feat: add x-twitter source packet skill

[kriptoburak]

Summary

• Adds x-twitter-source-packets as a utility skill for traceable public X/Twitter evidence packets.
• Registers the skill in the README and Claude plugin marketplace metadata.
• Rebuilds the per-plugin distribution files for the new skill.

Validation

• JSON validation for .claude-plugin/marketplace.json passed.
• Plugin build and dry-run build both completed successfully.
• Git whitespace validation passed.
• Markdown link checks passed for README.md and the new skill files.