docs: add mailbox-first collaboration model

CHANGELOG.md

@@ -2,6 +2,34 @@
 
 All notable changes to this repository are tracked here.
 
+## 2026-04-14
+
+- Commit hash: pending (next commit)
+- Added a mailbox-first coordination model so actionable minion communication
+  moves into packet directories while `minions/chat/` becomes a PM-owned
+  summary surface in:
+  - `README.md`
+  - `INIT.md`
+  - `MEMORY.md`
+  - `docs/collaboration-playbook.md`
+  - `docs/downstream-onboarding-playbook.md`
+  - `docs/downstream-upgrade-playbook.md`
+  - `docs/export-manifest.md`
+  - `docs/operator-onboarding-checklist.md`
+  - `minions/README.md`
+  - `minions/chat/README.md`
+  - `minions/chat/general-thread-template.md`
+  - `minions/chat/topic-thread-template.md`
+  - `minions/plans/milestone-plan-template.md`
+- Added template-managed mailbox assets in:
+  - `docs/project/mailbox-collaboration-model.md`
+  - `minions/mail/README.md`
+  - `minions/mail/packet-template.md`
+- Clarified staged rollout and downstream export behavior so legacy chat packets
+  may finish in place, new follow-up packets move to mail, and template packet
+  history is not exported into downstream repos
+- Bumped template version to `1.4.0-1.0.0` in `minion-version.md`
+
 ## 2026-04-12
 
 - Commit hash: pending (next commit)