* fix(imessage): only warn about empty group allowlist when messages actually drop
With groupPolicy="allowlist", a non-empty effective groupAllowFrom admits
group messages even when channels.imessage.groups is empty (senderFilterBypass
in src/config/group-policy.ts), so the startup warning "Every inbound group
message will be dropped" fired as a false positive for that configuration.
The warning now mirrors the runtime gate's effective sender allowlist (same
allowFrom fallback + legacy chat-target merge) and fires only when both the
groups registry and the effective group sender allowlist are empty - the only
startup-provable drop-all config. The message now names groupAllowFrom as the
fix, since adding groups entries alone leaves the sender gate blocking.
Docs: describe the two group gates' warnings separately with per-warning
remedies instead of implying both fire for the same config.
* docs(imessage): format migration guide
* fix(imessage): warn on empty group sender allowlist
* docs: move iMessage fix to unreleased