refactor(discovery): extract blindspot mode from explore into its own skill

[kyle-sexton]

Blindspot mode builds the USER's knowledge with a distinct output contract (prompt-fix cards, no EXPLORE.md, outcome gate skipped) — a grafted second responsibility on explore.

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/discovery/skills/explore/SKILL.md (remove blindspot-mode branch + its output-contract description), plugins/discovery/README.md (drop the blindspot mention, add the new sibling skill), new skill directory under plugins/discovery/skills/ (or wherever the naming pass lands) carrying blindspot's existing prompt-fix-card contract verbatim. No design decision — the output contract already fully described in the issue body.