Channels design: instruction bridge for multi-transport gateway

Design note for a channel instruction bridge on top of the existing gateway layer.

Scope:

• Normalize inbound channel messages into a transport-neutral instruction envelope
• Route instructions through the existing agent runner
• Send responses back over the same channel
• Keep channel-specific auth, allowlists, and formatting in the gateway layer

Suggested next steps:

• Define the instruction contract
• Add normalization/session-resolution tests
• Implement outbound reply handling for WhatsApp first

Source note was CHANNELS_DESIGN.md, which is being removed from the worktree after issue creation.