fix: prevent multi-channel response routing race condition (#4530)

## Problem

When multiple channels (WhatsApp, iMessage, Telegram, etc.) are active,
responses intended for one channel could leak to another channel if
messages arrived simultaneously. This created serious privacy concerns.

**Root Cause:**
All channels were updating the SAME main session key with their delivery
context (channel, to, accountId). When messages arrived concurrently:

1. WhatsApp message arrives → updates session['agent:main:main'].lastChannel = 'whatsapp'
2. Agent starts processing (takes time)
3. iMessage message arrives → OVERWRITES session['agent:main:main'].lastChannel = 'imessage'
4. WhatsApp response delivers → reads session lastChannel = 'imessage'
5. Response goes to wrong channel!

## Solution

Changed all channel monitors to use the channel-specific session key
(route.sessionKey) instead of the shared main session key
(route.mainSessionKey) when updating delivery context.

Now each channel updates its own isolated session:
- WhatsApp: session['agent:main:whatsapp:dm:+1234']
- iMessage: session['agent:main:imessage:dm:alice']
- Telegram: session['agent:main:telegram:dm:12345']

This prevents cross-channel state clobbering.

## Changes

- src/discord/monitor/message-handler.process.ts
- src/imessage/monitor/monitor-provider.ts
- src/line/bot-message-context.ts
- src/signal/monitor/event-handler.ts
- src/slack/monitor/message-handler/dispatch.ts
- src/slack/monitor/message-handler/prepare.ts
- src/telegram/bot-message-context.ts
- src/web/auto-reply/monitor/process-message.ts

All changed from:
  sessionKey: route.mainSessionKey

To:
  sessionKey: route.sessionKey

## Testing

Manual testing needed:
1. Configure 2+ channels (e.g., WhatsApp + iMessage)
2. Send message to channel A
3. Immediately send message to channel B (within 100ms)
4. Verify responses go to correct channels

Fixes #4530

src/discord/monitor/message-handler.process.ts

@@ -302,7 +302,7 @@ export async function processDiscordMessage(ctx: DiscordMessagePreflightContext)
     ctx: ctxPayload,
     updateLastRoute: isDirectMessage
       ? {
-          sessionKey: route.mainSessionKey,
+          sessionKey: route.sessionKey,
           channel: "discord",
           to: `user:${author.id}`,
           accountId: route.accountId,

src/imessage/monitor/monitor-provider.ts

@@ -510,7 +510,7 @@ export async function monitorIMessageProvider(opts: MonitorIMessageOpts = {}): P
       updateLastRoute:
         !isGroup && updateTarget
           ? {
-              sessionKey: route.mainSessionKey,
+              sessionKey: route.sessionKey,
               channel: "imessage",
               to: updateTarget,
               accountId: route.accountId,

src/line/bot-message-context.ts

@@ -282,7 +282,7 @@ export async function buildLineMessageContext(params: BuildLineMessageContextPar
   if (!isGroup) {
     await updateLastRoute({
       storePath,
-      sessionKey: route.mainSessionKey,
+      sessionKey: route.sessionKey,
       deliveryContext: {
         channel: "line",
         to: userId ?? peerId,
@@ -432,7 +432,7 @@ export async function buildLinePostbackContext(params: {
   if (!isGroup) {
     await updateLastRoute({
       storePath,
-      sessionKey: route.mainSessionKey,
+      sessionKey: route.sessionKey,
       deliveryContext: {
         channel: "line",
         to: userId ?? peerId,

src/signal/monitor/event-handler.ts

@@ -156,7 +156,7 @@ export function createSignalEventHandler(deps: SignalEventHandlerDeps) {
       ctx: ctxPayload,
       updateLastRoute: !entry.isGroup
         ? {
-            sessionKey: route.mainSessionKey,
+            sessionKey: route.sessionKey,
             channel: "signal",
             to: entry.senderRecipient,
             accountId: route.accountId,

src/slack/monitor/message-handler/dispatch.ts

@@ -27,7 +27,7 @@ export async function dispatchPreparedSlackMessage(prepared: PreparedSlackMessag
     });
     await updateLastRoute({
       storePath,
-      sessionKey: route.mainSessionKey,
+      sessionKey: route.sessionKey,
       deliveryContext: {
         channel: "slack",
         to: `user:${message.user}`,

src/slack/monitor/message-handler/prepare.ts

@@ -533,7 +533,7 @@ export async function prepareSlackMessage(params: {
     ctx: ctxPayload,
     updateLastRoute: isDirectMessage
       ? {
-          sessionKey: route.mainSessionKey,
+          sessionKey: route.sessionKey,
           channel: "slack",
           to: `user:${message.user}`,
           accountId: route.accountId,

src/telegram/bot-message-context.ts

@@ -617,7 +617,7 @@ export const buildTelegramMessageContext = async ({
     ctx: ctxPayload,
     updateLastRoute: !isGroup
       ? {
-          sessionKey: route.mainSessionKey,
+          sessionKey: route.sessionKey,
           channel: "telegram",
           to: String(chatId),
           accountId: route.accountId,

src/web/auto-reply/monitor/process-message.ts

@@ -295,7 +295,7 @@ export async function processMessage(params: {
       cfg: params.cfg,
       backgroundTasks: params.backgroundTasks,
       storeAgentId: params.route.agentId,
-      sessionKey: params.route.mainSessionKey,
+      sessionKey: params.route.sessionKey,
       channel: "whatsapp",
       to: dmRouteTarget,
       accountId: params.route.accountId,