fix(whatsapp): mark DMs as isMention so router auto-engages

DMs are addressed to the bot by definition. Mirror the chat-sdk-bridge
semantics (onDirectMessage forwards isMention=true) so the host router
can short-circuit DM auto-create + approval flow at router.ts:184
instead of silently dropping inbound from chats it doesn't recognize.

Without this, a v1 → v2 migration where Baileys' LID→phone translation
fails on the first message (no senderPn, signal repository cold) leaves
v2 unable to route DMs at all — the chatJid arrives as `<lid>@lid`,
the migrated messaging_group is keyed by `<phone>@s.whatsapp.net`, and
the router drops because !isMention.

Group messages keep isMention undefined so the router falls through to
agent-name regex matching, unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/channels/whatsapp.ts

@@ -571,6 +571,13 @@ registerChannelAdapter('whatsapp', {
             const inbound: InboundMessage = {
               id: msg.key.id || `wa-${Date.now()}`,
               kind: 'chat',
+              // DMs are addressed to the bot by definition. Mark them as
+              // platform-confirmed mentions so the router treats them like
+              // chat-sdk-bridge's onDirectMessage path — auto-creating an
+              // approval-required messaging_group when the chat is unknown,
+              // instead of silently dropping at router.ts:184.
+              isMention: !isGroup ? true : undefined,
+              isGroup,
               content: {
                 text: content,
                 sender,