chore: add PR and issue templates

Structured intake for contributors. PR template enforces atomic
per-lesson commits, markdown-link ROADMAP rows, no code comments,
built-from-scratch-first. Issue templates for bug reports and new
lesson proposals give contributors a clear starting point.

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Something in a lesson or on the site is wrong
+title: "[bug] "
+labels: bug
+---
+
+## Where
+
+- Phase / lesson: <!-- e.g. Phase 4 · 06-object-detection-yolo -->
+- File / URL: <!-- e.g. phases/04-computer-vision/06-object-detection-yolo/code/main.py or aiengineeringfromscratch.com/lesson.html?path=... -->
+
+## What's wrong
+
+<!-- One-paragraph description. What did you expect vs. what you saw. -->
+
+## Reproduce
+
+1.
+2.
+3.
+
+## Environment
+
+- OS:
+- Python / Node / other runtime version:
+- How you ran it (local, Colab, Docker, etc.):
+
+## Screenshot or logs
+
+<!-- Drop a screenshot or paste the traceback if you have one. -->

.github/ISSUE_TEMPLATE/new_lesson_proposal.md

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+---
+name: New lesson proposal
+about: Pitch a lesson before writing it
+title: "[lesson] Phase NN · "
+labels: new-lesson
+---
+
+## Lesson
+
+- Phase: <!-- e.g. Phase 5 (NLP) -->
+- Proposed number: <!-- e.g. 03 (lessons are numbered within a phase) -->
+- Working title:
+- Type: Build | Learn
+- Languages: <!-- Python, TypeScript, Rust, Julia -->
+- Estimated time: <!-- e.g. ~75 min -->
+
+## Why it belongs
+
+<!-- Two or three sentences. What can't a learner do without this lesson? Why here in the sequence? -->
+
+## Prerequisites
+
+<!-- Which prior lessons does this depend on? -->
+
+## What the learner ships
+
+<!-- Every lesson produces a reusable artifact. Which one? -->
+
+- [ ] Prompt
+- [ ] Skill
+- [ ] Agent
+- [ ] MCP server
+- [ ] Tool / script
+
+## Outline
+
+1. **The Problem** — what pain does this solve?
+2. **The Concept** — core mental model, no code yet
+3. **Build It** — from-scratch implementation plan
+4. **Use It** — which framework / library to show after
+5. **Ship It** — the artifact to produce
+6. **Exercises** — 3 proposed
+
+## Open questions
+
+<!-- Anything you want input on before starting. -->

.github/PULL_REQUEST_TEMPLATE.md

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+<!-- Thanks for contributing. Fill out what applies. Delete sections that don't. -->
+
+## What this PR does
+
+<!-- One-sentence summary. -->
+
+## Kind of change
+
+- [ ] New lesson
+- [ ] Fix to an existing lesson
+- [ ] Translation
+- [ ] New output (prompt, skill, agent, MCP server)
+- [ ] Docs / website / tooling
+
+## Checklist
+
+- [ ] Code runs without errors with the listed dependencies
+- [ ] No comments in code files (docs explain, code is self-explanatory)
+- [ ] Built from scratch first, then shown with a framework (for new lessons)
+- [ ] Lesson folder matches `LESSON_TEMPLATE.md` structure
+- [ ] ROADMAP.md row for the lesson is a markdown link (`[Name](phases/...)`), not bare text
+- [ ] One lesson per commit (atomic per-lesson rule)
+- [ ] Tested locally / code output matches what `docs/en.md` claims
+
+## Phase / lesson
+
+<!-- e.g. Phase 5 · 03-tokenizers -->
+
+## Notes for reviewer
+
+<!-- Anything surprising, any deviations from the template, open questions. -->