RFC: Add @flue/channels.

[cpojer]

This PR builds on top of the earlier explorations from today of using Chat SDK inside of fileameeting, and then on top PR #233 which integrates agents/chat-sdk for durable object storage for flue used together with Chat SDK on Cloudflare.

The initial version only supports Slack, and is an RFC because it requires extensive deployment/production testing. We should also build more channel integrations to validate the design.

@FredKSchott For this PR, would you mind checking out the docs and letting me know if it is directionally good?

[FredKSchott]

Very useful exploration! Two issues that I'm aware of:

• Channels could be multi-agent, the 1:1 connection of channel {agent ...} is likely not the right coupling.
• If Channel is 1:1 with agent (and you solve multi-agent by just defining multiple channels) then it should live in the agent TS file. Not everyone has an app.ts. But then that breaks how some channels work where they assume a single shared endpoint.
• I'm surprised we'd need a different integration story for Cloudflare vs. Node. Because we're using Hono as foundational server, we should be able to do a single integration across both CF and Node?

I have an old plan for channels that actually explored a lot of this, so don't need to start from square 1 if we want to pick back up from that RFC: #237

that's ready to go for an agent to implement. Maybe I'll just pass it along and see what that PR looks like and we can compare. If you have any feedback as someone who's built something with Slack and Flue, please let me know!

[cpojer]

I changed this RFC to build @flue/channels based on your previous RFC and the issues you pointed out in the initial version. It should cover everything (besides a Discord channel), and likely requires more end-to-end testing before merging. Is there anyone who could help with this?