Every skill now carries marketplace metadata

Marketplace metadata backfill

All 613 SKILL.md files (team source plus the root mirror) now carry compatibility and tags frontmatter fields. Tags are drawn from a per-agent domain table plus the skill's own name, so identical skills always get identical tags across every agent that ships one.

The generator behind the backfill, scripts/gen-skill-metadata.py, is idempotent (safe to re-run whenever a new skill is added) and writes atomically via temp-file-plus-rename, so a crash mid-run can't truncate a file. It fails loudly instead of guessing if a new agent shows up without a tag mapping.

Fixed

• -README version badge was pinned at 1.2.0, nine releases behind.
• -tonone-onboard, the one cross-agent skill with no owning agent, got a degenerate single-word tag from the new generator. It now has a proper fallback tag set.