docs: add mailbox-first collaboration model

docs/downstream-onboarding-playbook.md

@@ -66,15 +66,23 @@ clone.
 6. Treat downstream-owned files as project surfaces, not template clones:
    - project `README.md`
    - live plans
-   - live chat history
+   - live mail packet history
+   - live chat summary history
    - downstream `CHANGELOG.md`
    - `ROADMAP.md`
    - `TODO.md`
 7. Run the onboarding checklist with the Operator and fill in project-specific
    decisions.
-8. Commit the vendored snapshot and exported live files together so the repo has
+8. Bootstrap mailbox use in the live downstream repo:
+   - read `MEMORY.md`
+   - read `docs/project/mailbox-collaboration-model.md`
+   - read `minions/mail/README.md`
+   - read `minions/mail/packet-template.md`
+   - use `minions/mail/` for new actionable packets
+   - use `minions/chat/` for PM daily summaries
+9. Commit the vendored snapshot and exported live files together so the repo has
    a clear starting baseline.
-9. After onboarding is approved, future template changes should use
+10. After onboarding is approved, future template changes should use
    `docs/downstream-upgrade-playbook.md`.
 
 ## Manual-Merge Guidance
@@ -109,5 +117,6 @@ clone.
 - files exported directly from template
 - files manually merged for downstream use
 - files intentionally left downstream-owned
+- confirmation that mailbox bootstrap docs were reviewed and `minions/mail/` is the active packet surface for new work
 - open Operator decisions
 - first commit scope and next owner