fix(agent): reject send tool when targeting the same conversation

Pass replyTarget through the full pipeline (ChatRequest → gateway
identity → agent headers → MCP session) so the send tool can detect
when the destination matches the current conversation and return an
error guiding the agent to reply directly instead.

internal/conversation/flow/resolver.go

@@ -161,6 +161,7 @@ type gatewayIdentity struct {
 	ChannelIdentityID string `json:"channelIdentityId"`
 	DisplayName       string `json:"displayName"`
 	CurrentPlatform   string `json:"currentPlatform,omitempty"`
+	ReplyTarget       string `json:"replyTarget,omitempty"`
 	ConversationType  string `json:"conversationType,omitempty"`
 	SessionToken      string `json:"sessionToken,omitempty"` //nolint:gosec // intentional: session token forwarded to agent gateway for channel reply routing
 }
@@ -470,6 +471,7 @@ func (r *Resolver) resolve(ctx context.Context, req conversation.ChatRequest) (r
 			ChannelIdentityID: strings.TrimSpace(req.SourceChannelIdentityID),
 			DisplayName:       displayName,
 			CurrentPlatform:   req.CurrentChannel,
+			ReplyTarget:       strings.TrimSpace(req.ReplyTarget),
 			ConversationType:  strings.TrimSpace(req.ConversationType),
 			SessionToken:      req.ChatToken,
 		},