feat(whatsapp): add text-based command menu

[reski-rukmantiyo]

Summary

• Add command menu system to the WhatsApp channel (#menu, #help, #stop, #stopall, #reset)
• Text-based approach — WhatsApp silently drops ListMessage from linked/companion devices (whatsmeow connects as linked device, not Business API)
• Commands are case-insensitive, non-command messages pass through to agent pipeline unchanged
• Reuses the existing PublishInbound + Metadata["command"] pattern so the gateway consumer works without modification

Closes #768

Changes

• New: internal/channels/whatsapp/commands.go — command detection, handling, text menu, command publishing
• Modified: internal/channels/whatsapp/inbound.go — command interception before agent pipeline

Test plan

• Send #menu or #help to WhatsApp bot → verify text menu appears
• Send #stop while task running → verify task is stopped and feedback received
• Send #stopall while tasks running → verify all tasks stopped and feedback received
• Send #reset → verify conversation history cleared and confirmation received
• Send normal text → verify it still goes to agent pipeline
• Verify commands work in both DMs and groups

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