Add lesson template for contributors

Standardized format for folder structure, documentation, code files,
and output artifacts. Includes the one-line motto, problem-concept-build-
use-ship pattern, and exercise guidelines.

LESSON_TEMPLATE.md

@@ -0,0 +1,133 @@
+# Lesson Template
+
+Use this template when creating a new lesson. Copy the folder structure and fill in the content.
+
+## Folder Structure
+
+```
+NN-lesson-name/
+├── code/
+│   ├── main.py            (primary implementation)
+│   ├── main.ts            (TypeScript version, if applicable)
+│   ├── main.rs            (Rust version, if applicable)
+│   └── main.jl            (Julia version, if applicable)
+├── notebook/
+│   └── lesson.ipynb       (Jupyter notebook for experimentation)
+├── docs/
+│   └── en.md              (lesson documentation)
+└── outputs/
+    ├── prompt-*.md         (prompts produced by this lesson)
+    └── skill-*.md          (skills produced by this lesson)
+```
+
+## Documentation Format (docs/en.md)
+
+```markdown
+# [Lesson Title]
+
+> [One-line motto — the core idea that sticks]
+
+**Type:** Build | Learn
+**Languages:** Python, TypeScript, Rust, Julia (list what's used)
+**Prerequisites:** [List prior lessons needed]
+**Time:** ~[estimated time] minutes
+
+## The Problem
+
+[2-3 paragraphs. What can't you do without this? Why should you care?
+Make it concrete — show a scenario where not knowing this hurts.]
+
+## The Concept
+
+[Explain with diagrams and intuition. No code yet.
+Use ASCII diagrams, tables, or link to visuals in the web app.
+Build mental models before implementation.]
+
+## Build It
+
+[Step-by-step implementation from scratch.
+Start with the simplest version, then add complexity.
+Every code block should be runnable on its own.]
+
+### Step 1: [Name]
+
+[Explanation]
+
+    [code block]
+
+### Step 2: [Name]
+
+[Explanation]
+
+    [code block]
+
+[...continue...]
+
+## Use It
+
+[Now show how frameworks/libraries do the same thing.
+Compare your from-scratch version to the library version.
+This proves the concept and introduces practical tools.]
+
+## Ship It
+
+[What reusable artifact does this lesson produce?
+Could be a prompt, a skill, an agent, an MCP server, or a tool.
+Include it here and save it in the outputs/ folder.]
+
+## Exercises
+
+1. [Easy — reinforce the core concept]
+2. [Medium — apply it to a different problem]
+3. [Hard — extend or combine with prior lessons]
+
+## Key Terms
+
+| Term | What people say | What it actually means |
+|------|----------------|----------------------|
+| [term] | [common misconception] | [actual definition] |
+
+## Further Reading
+
+- [Resource 1](url) — [why it's worth reading]
+- [Resource 2](url) — [why it's worth reading]
+```
+
+## Code File Guidelines
+
+- Code must run without errors
+- No comments — code should be self-explanatory
+- Use the language that fits best for the topic
+- Include a `requirements.txt` or equivalent if there are dependencies
+- Start simple, build up complexity
+- Every function and class should have a clear purpose
+
+## Output File Format
+
+### Prompts
+
+```markdown
+---
+name: prompt-name
+description: What this prompt does
+phase: [phase number]
+lesson: [lesson number]
+---
+
+[Prompt content]
+```
+
+### Skills
+
+```markdown
+---
+name: skill-name
+description: What this skill teaches
+version: 1.0.0
+phase: [phase number]
+lesson: [lesson number]
+tags: [relevant, tags]
+---
+
+[Skill content]
+```