Use a plan.md File as a Checkpoint That Survives Context Resets

Context windows run out. Sessions crash. You close a tab by accident. When that happens mid-task, you lose everything and have to brief Claude from scratch.

The solution is a plan.md file written before Claude starts building. Keep it simple:

# Add JWT Authentication

## Goal
Add token-based auth to the Express API.

## Steps
- [x] Install jsonwebtoken and bcrypt
- [ ] Create /auth/login and /auth/register routes
- [ ] Add auth middleware to protect existing routes
- [ ] Write integration tests

## Files to touch
- src/routes/auth.ts (new)
- src/middleware/auth.ts (new)
- src/app.ts (register routes)

As Claude completes each step, it checks the box. If the session dies halfway through, start a fresh one and point it at the plan:

Here is the current plan.md. Continue from the first unchecked step.

Claude picks up exactly where it left off. No re-briefing, no repeated context-setting, no trying to reconstruct where things were up to.

This also works when handing a task between parallel sessions. One session writes the plan, another executes it. The plan.md is the shared contract between them.

This is distinct from Claude Code's built-in Plan Mode (Shift+Tab) — that's for the planning conversation itself. A plan.md is a persistent artefact on disk that outlives any single session.

The plan.md is not just a checklist — it is the memory that keeps long work on track.