teach: add high-level "How <topic> works" overview before lessons (#30)

• teach: open the lesson with a high-level "How <topic> works" overview

The lesson template jumped from "What you'll learn" straight into checkpoint-anchored lessons, leaving readers without a picture of the system itself. Add a "How <topic> works" section before the lessons: what it does, the moving parts, the end-to-end lifecycle (now the home for the optional Mermaid diagram), and the key design idea.

Also distinguish it from "Mental model" (system vs how the team reasons about it) and require the overview to stay grounded in what the transcripts support rather than inventing architecture.

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com

• teach: drop "What you'll learn" and "Mental model"; bump version to 0.7.0

The "How <topic> works" overview now carries the lesson's framing, making the "What you'll learn" preamble and the separate "Mental model" section redundant. The lesson goes straight from the system overview into the checkpoint-anchored lessons.

Bump all plugin manifests (claude, codex, cursor, gemini, npm, marketplace) from 0.6.0 to 0.7.0.

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com

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