#57 Add a skill proposing the top 3 next features for Goldener

Summary by cubic

Adds a propose-goldener-features skill that turns the research in this repo into the top 3 feature proposals for Goldener, each with an implementation plan.

• Registers the skill in the skills marketplace.
• The skill reads Goldener's main branch and its existing issues to avoid duplicates, grounds candidates in IDEAS.md and BIBLIOGRAPHY.md, and ranks them on value, grounding, gap and feasibility.
• Issues are created only after the user explicitly approves each proposal one at a time; the skill never edits repos or runs code from Goldener's clone.