Onboarding docs + handoff guide

[henry0816191]

Summary

The project has bus factor 1. Write onboarding documentation and a handoff guide sufficient for a second maintainer to operate, debug, and extend the service without prior context.

Background

The eval notes that engineering quality is high but organizational resilience is the primary concern. A 24/7 monitoring service with no onboarding path is operationally fragile regardless of code quality. The README must satisfy the acceptance criterion: "a newcomer can set up and run the service without internal wiki access."

Tasks

Onboarding doc (docs/onboarding.md or expanded README):

• Repo layout — what each of the 8 source modules does in one sentence
• Local dev setup — venv, env vars, PostgreSQL, running tests
• Deployment steps — Docker build, docker-compose, nginx config
• Environment variables reference — every key in .env.example explained
• Scheduling mechanism — how the asyncio-based Scheduler works, poll intervals

Handoff guide (docs/handoff.md):

• Non-obvious design decisions (e.g., why two-frequency hot/cold probing; why HEAD-only for D-papers)
• Known operational gotchas (rate limits, isocpp.org stability, Slack 429 behaviour)
• Open TODOs and deferred items from the eval
• Who to contact for Slack token / PostgreSQL credentials

Acceptance Criteria

• A developer unfamiliar with the codebase can run the full test suite and start the bot locally using only the docs
• Handoff section covers at least 3 non-obvious decisions
• PR reviewed by Will before Friday demo