docs: add mailbox-first collaboration model

minions/mail/packet-template.md

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+# Packet Template
+
+Use this as the default structure for a new mailbox packet.
+
+Recommended path:
+
+`minions/mail/YYYY-MM-DD-sender-to-recipient-topic/`
+
+## `request.md`
+
+```text
+TARGET ROLE:
+
+DECISION:
+
+RATIONALE:
+
+ACTION NEEDED:
+
+### AM
+
+No action needed yet.
+
+### CM
+
+No action needed yet.
+
+### SM
+
+No action needed yet.
+
+### OM-Test / OM
+
+No action needed yet.
+
+### Operator
+
+No action needed until the response is reviewed.
+```
+
+## `response.md`
+
+```text
+DECISION:
+
+RATIONALE:
+
+SCOPE COMPLETED:
+
+OUT OF SCOPE:
+
+EVIDENCE:
+
+BLOCKERS/RISKS:
+
+ACTION NEEDED:
+
+NEXT OWNER:
+
+READY CHECK:
+```
+
+## `verdict.md` (optional)
+
+```text
+DECISION:
+
+RATIONALE:
+
+ACTION NEEDED:
+
+NEXT OWNER:
+
+READY CHECK:
+```