Make Codex the primary tutor and add beginner LLM foundations

[Doppp]

Summary

• add an opt-in tutor-led course profile where coding-agent conversation leads and the browser focuses on checkpoints, explorables, evidence, exercises, and collapsed reference notes
• expose provider-neutral active tutor state and update Codex, Claude, portable skill, scaffold, PRD, architecture, and authoring contracts
• add beginner Generative AI/LLM terminology and next-token-loop lessons with two tested accessible explorables before the existing training sequence
• keep ordered-list markers inside sandbox panels at narrow widths

Verification

• clean worktree: pnpm check (36 suites, 109 tests)
• pnpm build
• pnpm course:test (20 suites, 61 model tests; 14 starter/reference pairs)
• pnpm test:browser (17 tests, including axe and 320px)
• pnpm site:test (2 tests)

The four unrelated learner exercise edits in the primary workspace were preserved and are not part of this PR.