EditMessage authorized the non-DM path with permissions.SendMessages alone
while every sibling message sink requires ReadMessages plus the mutate bit, so
a user denied READ_MESSAGES could still rewrite an old post and have the edit
broadcast to the channel. The edit gate now calls the existing
checkSendPermission helper and collapses its error into the sink's
pre-existing opaque ErrForbidden, so the reply stays a non-oracle.
Verified by a panel of agents; the added test fails against the unpatched
tree, showing the edit succeeded before the fix.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
