Rewrite README as public-facing intro

[StewAlexander-com]

Summary

The previous README read like a code walkthrough — long bullet lists, internal
APIs, deep sandbox config, milestone tables — before ever telling a new visitor
what the project actually is. This rewrite re-orients it as the public's
introduction to the project.

What's new

• Lead with the pitch. First screenful answers what is this? and why should I care? — private, offline Python tutor that runs entirely on your machine.
• A who-it's-for table for self-learners, educators, privacy-minded teams, tinkerers.
• A feature/value table instead of a 7-bullet "Goals" list.
• Two Mermaid diagrams kept (system + teaching loop) plus an ASCII architecture sketch — visuals over words.
• Quick start trimmed to two commands with one callout about opt-in y/N prompts. Full env-var matrix and CI/non-interactive recipes moved out to docs/install-runtime-workflow.md, which already covers them.
• Safety section reframed as a side-by-side "in force / not in force" table, explicitly called prototype safety, not production isolation — no overclaiming.
• Low-level detail removed from the README (full env-var table, endpoint catalogue, repository tree, "Minimal API shape", "Example LLM call", "Teaching loop" enumeration, milestone list, "Build philosophy"). It all still lives in docs/, backend/README.md, frontend/README.md, and curriculum/exercises/README.md, which the README now links to from a single "Going deeper" list.
• Concise CI mention pointing at the existing workflow.

What's preserved

• Offline / local-Gemma-via-Ollama positioning.
• FastAPI backend + static PWA frontend + inline code lab.
• Structured graded exercises with visible + hidden tests.
• Curated docs allowlist and no-LLM-URL-generation guarantee.
• Prototype sandbox controls (AST scan, isolated subprocess, timeouts, rlimits, env scrub).
• All install/run commands (./install.sh, ./run.sh, opt-in prompts).

File changes

• README.md — 594 → 193 lines (-452 / +142). No other files touched.

Testing

• Verified every internal link target exists (docs/*, frontend/README.md, backend/README.md, curriculum/exercises/README.md, backend/app/safety.py, backend/app/docs_refs.py, .github/workflows/ci.yml, adr/0001-offline-first-local-llm.md, prompts/tutor-system-prompt.md).
• No code changes — backend pytest / frontend syntax / shell-lint jobs are unaffected.
• Visual check of Mermaid rendering on the PR page.

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