docs: record kept-split mailbox architecture decision (2026-06-21)

/mailbox (ACG own-mail, single-tenant 1873b1b0) and client send (suite
Exchange Operator b43e7342, multi-tenant) stay separate on purpose: 1873b1b0
is single-tenant so it cannot serve clients; consolidating onto exchange-op was
rejected (privilege creep on casual own-mail + loses Contacts). Documented the
why in commands/mailbox.md scope boundary + feedback memory so it stops being
re-litigated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.claude/commands/mailbox.md

@@ -21,7 +21,7 @@ Read and send mail for an Arizona Computer Guru mailbox via Microsoft Graph, usi
 
 Microsoft Graph access to ACG's own mailboxes (azcomputerguru.com tenant). Reading is unrestricted; **sending and replying are always gated** behind a full draft preview + explicit confirmation. Sends go out *as* the mailbox owner (your `From:`), saved to your Sent Items.
 
-**Scope boundary:** this is for ACG's OWN mailboxes (the dedicated mailbox app, azcomputerguru.com only). For reading a CLIENT tenant's mailboxes (breach checks, rule audits), use `/remediation-tool` — the tiered Security Investigator / Exchange Operator apps, different apps and purpose.
+**Scope boundary (by design — kept split, decided 2026-06-21):** this is for ACG's OWN mailboxes only. The ComputerGuru Mailbox app (`1873b1b0`) is **single-tenant (azcomputerguru.com)** — it has no service principal in client tenants, so `/mailbox` structurally cannot read or send for clients. For a CLIENT tenant's mail (breach checks, rule audits, IR victim-notification sends), use `/remediation-tool` — the multi-tenant suite, where **Exchange Operator (`b43e7342`) holds `Mail.Send`**. The two stay separate on purpose: own-mail rides a least-privilege ACG-only app, not the suite's tenant-wide EXO-write app.
 
 ## API Configuration