RFD 3102: Contributor onboarding checklist automation

[lowlydba]

New contributors currently bounce between README sections, old issue comments, and tribal knowledge in chat. The idea is to automate a first-week checklist that opens when a contributor has their first PR merged. It would include local setup validation, test commands, style conventions, and links to recent architectural notes. We should pilot this with a small group of external contributors and track time-to-second-PR plus first-review turnaround. If signal is positive, we can expand templates by role, such as docs-first or code-first contributor tracks.