education: add explain skill — one-shot plain-language altitude drop with teach handoff

[kyle-sexton]

Sibling to education:teach (coaching = multi-session; explain = one-shot). Imperative-verb name per the codified naming grammar.

Design (locked in triage interview 2026-07-17):

• Core move: explain the given thing (concept, code, error, architecture, prior assistant output) in genuinely plain words — concrete analogy, zero jargon — then layer altitude up on request (high-school level, peer level).
• Empty argument = anaphora default: the object is the assistant's previous output, so "I don't get it" needs no topic named.
• Description must carry colloquial trigger phrasings for model auto-invocation: "I don't understand this", "I don't get it", "what does this actually do/mean", "explain simply", "ELI5", "rephrase that". Auto-trigger is best-effort; explicit invocation is the guaranteed path.
• Feynman gap check: when the explanation cannot shed jargon, that is a detected understanding gap — the skill surfaces it instead of papering over it (comprehension self-test, not a rewording service).
• Handoff: closes by offering /education:teach topic <x> for a multi-session deeper dive (same plugin, first-party reference).
• Tone anchor: Michael Scott's "explain this to me like I'm five" — quote/episode citation verified at build time, not from memory. The Feynman technique is the methodological grounding; the popular Einstein attribution of "if you can't explain it simply..." is shaky and is cited as Feynman-technique methodology instead.

[kyle-sexton]

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

Sites + fix pattern: new education:explain skill directory (SKILL.md carrying the design already locked in this issue's own triage interview — core plain-words move, anaphora default, auto-invocation trigger phrasings, Feynman gap check, /education:teach handoff), plugin manifest/marketplace registration for the new skill, education plugin README (add the sibling entry alongside teach). No open design decision — the interview this issue cites already locked every behavioral detail.