fix(slack): resolve Slack Connect sender names from inline user_profile

[meidad]

Summary

Slack Connect DMs were rendering in default-channel draft notifications as `Nomos DM from external Slack user (U073UDQAT0T)` instead of the sender's actual name.

Why this happened: `users.info` returns `user_not_found` for cross-workspace senders in our own workspace. The existing cross-workspace fallback only worked if we happened to have a token for the sender's home workspace — for most Slack Connect senders, we don't.

What Slack already gave us: every `message` event already carries the sender's display info inline as `user_profile` (same shape as `users.info`'s `profile`) plus their home `user_team`. We were ignoring both.

Change

src/daemon/channels/slack-user.ts — on every incoming message we now seed the username cache from `event.user_profile.real_name` (then `display_name`, then `name`) BEFORE any handler calls `lookupUserName`. We also remember `event.user_team` so that when we DO need a cross-workspace API call, we try that specific workspace's token first instead of scanning every workspace.

Won't overwrite a previously-resolved real name with an event-provided one (e.g., a name from `users.info` is preferred over the event payload). Only seeds when the cache is empty or holds the `external Slack user (...)` fallback label.

Side benefit: if `user_profile.email` matches the owner's email, we mark the foreign user_id as one of our own, so we skip drafting for messages "from us" arriving via Slack Connect.

Backstory

This commit was orphaned from #54 — I pushed it to that branch after the squash-merge already happened, so it never made it onto main. Same content, fresh branch.

Test plan

• Have a Slack Connect contact DM you. Verify the default-channel notification reads `Nomos DM from Their Real Name` instead of the U-id fallback.
• Have a regular intra-workspace user DM you. Verify their name still resolves correctly (no regression).
• Restart the daemon and confirm sender names persist via the cache across multiple messages from the same user.

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