docs: present the omnichannel architecture

[setkyar]

Summary

• reposition WATeamInbox as an omnichannel team inbox
• clearly separate currently supported WhatsApp linked-device and Telegram Bot providers from planned integrations
• redraw the README architecture around the channel-neutral spine and provider adapters
• update the technical overview, repository map, limitations, local Telegram guidance, and security notes

Current vs planned

Available today:

• WhatsApp linked device
• Telegram Bot API

Architectural direction, explicitly marked as planned:

• WhatsApp Cloud API
• email
• Messenger
• Instagram

Validation

• git diff --check
• Markdown fence balance check
• README Mermaid architecture rendered successfully with Mermaid CLI
• relative documentation links checked against the repository