fix(security): deny admin access on IM channels without admins config

Previously, when admins[channel] was not configured for an IM channel
(wechat, telegram, etc.), isAdminChatter returned true for ALL chatters,
allowing any anonymous user to modify SOUL.md, IDENTITY.md and other
identity files via write_file/edit_file tools.

Now falls back to ownership check (chatter UserID == agent ownerUserID)
instead of granting blanket admin. Operators can still explicitly grant
admin to specific platform IDs via admins[channel] in agent.json.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: idoubi

internal/agent/slash.go

@@ -174,10 +174,12 @@ func (a *Agent) isAdminChatter(msg bus.InboundMessage) bool {
 	}
 	list, ok := a.admins[msg.Channel]
 	if !ok || len(list) == 0 {
-		// No allowlist configured for this channel → preserve legacy
-		// unrestricted behavior. Operators opt in to group-chat
-		// protection by populating admins[channel].
-		return true
+		// No allowlist configured for this channel. Fall back to
+		// ownership check: if the IM chatter's resolved FastClaw
+		// user_id matches the agent owner, they're admin. Otherwise
+		// deny — an unconfigured allowlist should NOT grant admin
+		// to every anonymous chatter on a public-facing IM channel.
+		return msg.UserID != "" && msg.UserID == a.ownerUserID
 	}
 	for _, id := range list {
 		if id == msg.UserID {