Consolidate channel sources into channels-src/ by moving whatsapp from
channels/. Add on_status stubs to Slack and WhatsApp so their WASM
binaries export the function added in the latest WIT. Fix Slack's
emit_message call to pass by reference (API changed). Guard WASM webhook
server startup to skip when the HTTP channel already occupies port 8080.
Co-Authored-By: Claude Opus 4.6 <[email protected]>
