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.

packages/agent/src/types/agent.ts

@@ -9,6 +9,7 @@ export interface IdentityContext {
   channelIdentityId: string
   displayName: string
   currentPlatform?: string
+  replyTarget?: string
   conversationType?: string
   sessionToken?: string
 }

packages/agent/src/utils/headers.ts

@@ -17,6 +17,9 @@ export const buildIdentityHeaders = (identity: IdentityContext, auth: AgentAuthC
   if (identity.currentPlatform) {
     headers['X-Memoh-Current-Platform'] = identity.currentPlatform
   }
+  if (identity.replyTarget) {
+    headers['X-Memoh-Reply-Target'] = identity.replyTarget
+  }
   if (options?.isSubagent) {
     headers['X-Memoh-Is-Subagent'] = 'true'
   }