refactor(firecrawl): extract maintainer update/drift pipeline from the wrapper skill

[kyle-sexton]

UPSTREAM.md drift gates and scripts/update.sh are maintainer-facing; mirror the context7 lookup-vs-setup split (firecrawl:update or setup).

Origin: skill-name audit + interview, contract at docs/topics/shadowed-skill-renames/PLAN.md (PR #256).

[kyle-sexton]

This was generated by AI during triage (historic normalization, rule 8g).

Sites + fix pattern: plugins/firecrawl/skills/firecrawl/SKILL.md (drop maintainer update/drift steps), plugins/firecrawl/skills/firecrawl/UPSTREAM.md + context/update-flow.md (move under the new maintainer skill), plugins/firecrawl/README.md (add the split entry). Mirror context7's existing lookup-vs-setup split exactly — no new design decision, precedent already established in this repo.