Add Obsidian X research skill

[kriptoburak]

Summary

• Add obsidian-x-research, a read-only skill for saving public X/Twitter research into Obsidian notes.
• Add the skill to the README table.
• Keep Xquik opt-in by requiring already-configured local access and recording verification gaps.

Validation

• python3 frontmatter and description length check for every skills/*/SKILL.md
• jq . .claude-plugin/plugin.json
• jq . .claude-plugin/marketplace.json
• curl -fsI https://docs.xquik.com/api-reference/overview
• curl -fsI https://xquik.com/openapi.json
• git diff --check
• staged added-line dash scan