Write a codebase onboarding guide

Write a codebase onboarding guide.

Structure:
1. **The mental model** — what this system does, in the terms the team uses. Three or four sentences, including the two or three domain words that appear everywhere.
2. **Reading order** — five to eight files or modules, in order, with one line each on why it comes at that point. *Not a directory tour.* Someone who reads these should be able to follow a request end to end.
3. **Trace one path all the way through** — a single common operation from entry to storage to response. This teaches more than any diagram.
4. **Their first task**, broken into steps with which parts of the reading it needs.
5. **Where the surprises are** — the parts that do not work the way the naming suggests, the historical decisions that look wrong, the file everyone is afraid of. *This is the section that only exists in people's heads and is the most valuable thing here.*
6. Look it up when you need it — everything else, as a list, not explained.

Rules:
- Assume they can program and do not know this system. Do not explain the language.
- *Do not attempt completeness.* Say explicitly what this guide leaves out.
- Where I did not tell you something, mark it as a question for the team rather than guessing at how the code works.
- Name who to ask for what.