Add public X product-signal research skill

[kriptoburak]

Summary

• Add xquik-product-signal-research as a public JoomPulse companion workflow.
• Use JoomPulse for product, sales, price, seller, logistics, catalog, and buy-box evidence.
• Use public X evidence only for demand language, objections, comparisons, creators, and timing.
• Treat social posts as qualitative signals, not marketplace proof.
• Require explicit approval before monitors, extraction jobs, or other paid or external-state actions.
• Add the skill to the README catalog.

Validation

• scripts/check-public-safety.sh
• Skill Creator validation
• Markdown link check
• Frontmatter name and folder match
• Sensitive-content and untrusted-input review
• git diff --check

The branch is rebuilt directly on current upstream main and contains no private product data or unrelated changes.

[kriptoburak]

Rebuilt this PR on current main at c5ce350. The new skill uses JoomPulse for marketplace evidence and public X data only for qualitative demand language, objections, comparisons, creators, and timing. It requires approval before paid or external-state actions. Public-safety, Skill Creator, link, frontmatter, and diff checks pass.