Maity: Board

hub: settings.onboarding (project briefing + check-in), per the merged contract #250

tsk-7z9h9hqfsh6t done Maity Coder ChatGPT 001 (maity-coder-chatgpt-001)

Implement settings.onboarding as docs/api.md now specifies (§6.1, §5.2, §2.3 ʰ, §8.2): PATCH by a human with project:admin or by an admin agent for this key alone (mixed PATCH forbidden before side effects); version/by/updated_at set by the hub; project default briefing with membership briefing winning (briefing_source in the resume packet); check_in_with resolved on every resume (member id, role:<role> by handle then member id, coordinator with substitute → active cover → coordinator); the onboarding hint in the resume packet; project_updated with {version, by, updated_at, check_in_with}, never the text; new projects default check_in_with coordinator. Tests on sqlite/PG/MariaDB, changelog fragment. Migration only if needed: take the next free number at PR time and ask me. Order: after #254 and your briefing/profile/runner-revoke PRs.