Delete secrets:onboardbase from all 25 packages

Part of ALL-883. Onboardbase is fully decommissioned (Victor confirmed 2026-05-12 in #eng). Audit (run on main as of 2026-05-12): - 25 package.json files contain a `secrets:onboardbase` script entry - ZERO non-package.json callers (grep -rln 'secrets:onboardbase' --exclude-dir=node_modules --exclude-dir=.git . | grep -v package.json returned empty) Steps: 1. Branch: rob/all-883-remove-secrets-onboardbase off main 2. For each of the 25 package.json files: remove the secrets:onboardbase script entry only — leave secrets:check etc. untouched 3. Run yarn install at root to confirm no script ref breaks 4. CI: make sure no workflow references secrets:onboardbase (audit already confirms none) 5. Open PR titled 'ALL-883: Remove secrets:onboardbase (decommissioned)', body 'Fixes ALL-883' if final, else 'Part of ALL-883' Gated on: #10455 merging (so this lands AFTER the pattern is live). NOT gated by extraction work — this is just a delete.