docs: add ONBOARDING.md — the process side of joining the team

[qiuethan]

What this changes

Adds docs/ONBOARDING.md, the people-and-process side of joining as a contributor, plus one-line pointers in README.md and docs/DEVELOPMENT.md so it's discoverable from either direction.

Why

We're about to onboard devs. DEVELOPMENT.md covers the technical path and ACCESS.md covers credentials, but nothing covered how working here actually works: zone ownership expectations, how issues and the board work, PR review norms, the AI-tooling policy, and what's expected week to week. This doc fills that gap and repeats nothing technical — it links out instead.

Contents:

• First-week checklist (access → read → make check green → zone assignment → first merged PR)
• What owning a zone means: reviewer, issue triage, keeping the zone's docs honest
• Getting work: self-serve ready issues, the three label axes (type/*, area/*, blocked/ready), the blocked/ready automation, claim = assign yourself, and the zone-vs-area distinction (issues are labeled by area; the zone link is just where the work will land)
• PR norms: review-ready checklist, zone-owner reviews, 48h turnaround both directions, staging-first
• AI-assisted development: AGENTS.md is the agent contract, you own every line, and ask @qiuethan if you're missing an AI tool you want
• Time commitment: ~5 hrs/week baseline, 2-week quiet threshold before zones are reassigned, heads-ups keep zones held

Zone

docs, plus one pointer paragraph in root (README.md). Spans two zones because the README pointer is what makes the new doc discoverable; splitting a one-line pointer into its own PR would be noise.

How to verify

Read docs/ONBOARDING.md. Check the relative links resolve:

cd docs && grep -o '](\([^)#]*\)' ONBOARDING.md | sed 's/](//' | sort -u | \
  while read f; do [ -n "$f" ] && { [ -e "$f" ] && echo "OK $f" || echo "MISSING $f"; }; done

Expected: every line prints OK.

Checklist

• Branched off staging and targeting staging.
• Ran the test suite locally — docs-only change; no code touched.
• ruff — not applicable, no Python touched.
• Read and followed the relevant docs/CONTRIBUTING.md — docs-only change.
• Docs updated in this PR where the change makes them wrong — this PR is the docs change; README and DEVELOPMENT.md pointers included.

🤖 Generated with Claude Code

https://claude.ai/code/session_01E4Waeq792GHzy42X6JCeCJ