Workflows cannot be scoped to all channels — reaction-capture needs one copy per channel

## Summary

A workflow is bound to exactly one channel and only sees events in that channel. There is no way to create a workflow that watches every channel in a community, and no way to fake one short of duplicating the workflow per channel and hand-syncing the copies.

## Where this is enforced

Event dispatch looks up workflows by channel id and nothing else — `crates/buzz-relay/src/handlers/event.rs` passes the community and event into `workflow_engine.on_event`, and matching is scoped by the workflow's `channel_id`. Desktop's workflow form (`desktop/src/features/workflows/ui/workflowDefinition.ts`) offers a single channel selector with no "all channels" option.

## Why it matters

The natural shape for reaction-driven capture is workspace-wide: *react with ♻️ anywhere, the item lands in the triage channel.* That is one rule. Today it is N rules, one per channel, that drift the moment any of them is edited — and each new channel silently isn't covered until someone remembers to add it.

With six project channels this is already unmanageable by hand.

## Requested

An "all channels" / community-scoped workflow binding: `channel: "*"` in the YAML and an **All channels** option in the Desktop selector, with the workflow matching any event in the community the owner can see. Pairs directly with block/buzz#2979 — workspace-wide capture also needs the destination channel to differ from the trigger channel.