feat[users]: add dm-channel command for group DM channel discovery

[shhac]

Summary

There's currently no way to discover group DM channels or send messages to a group of users via DM. This adds agent-slack user dm-channel which resolves user handles/IDs to a DM or group DM channel ID via conversations.open, making it possible to then message that channel like any other.

This is a successor to #10, which was closed due to upstream repo structure changes before it could be merged.

Changes

• Add user dm-channel <users...> command (accepts 1-8 user IDs or @handles)
• Add getDmChannelForUsers to resolve users and open the conversation
• Add skill documentation (SKILL.md section + commands.md reference)

Test plan

• agent-slack user dm-channel @alice — returns 1:1 DM channel ID
• agent-slack user dm-channel @alice @bob — returns group DM channel ID
• agent-slack user dm-channel @alice @bob @charlie — returns group DM channel ID
• Verify returned channel IDs work with agent-slack message send <channel_id> "test"

[shhac]

@nwparker Thoughts on moving the location of this command to agent-slack channel get ?

With channel list landed it feels like this could be a sibling there.

Also happy to leave as is / for you to do the rename given these review cycle times

[AmethystLiang]

Hey @shhac — thanks for picking this up, the implementation looks clean.

On the naming question: I'd keep it under user rather than moving to channel get — that name should be reserved for looking up channel info by ID/name, and since the primary input here is user handles, user is the right home.

One thought: what about user dm-open instead of dm-channel? Since conversations.open creates the DM if it doesn't exist, open better conveys the side effect — dm-channel reads as purely read-only.

Happy to rebase this onto main and get it across the finish line. Let me know!

[shhac]

@AmethystLiang so when you're opening a group DM it would look like
agent-slack user dm-open @alice @bob @charlie => Cabc1234?

[shhac]

Changes made

[AmethystLiang]

Looks good — merged! Thanks for picking this back up Paul, clean implementation.