Desktop: tasks are project-scoped only, although every task records its origin channel

### Summary

Tasks are only reachable from the project workspace. A channel view has no task list of its own, even though every task already records the channel it originated in.

### What I observed

Desktop 0.5.22, self-hosted relay 0.2.1.

With a project that has several channels (`bugs`, `features`, `decisions`), tasks created from each channel all appear in one project-wide list. Opening `#bugs` gives no way to see "the tasks that belong to this channel" — the context rail with **Tasks / Reviews / Commits / Files / People** is rendered by `ProjectsView`, not by the channel view.

### The data is already there

`ProjectIssuesPanel.tsx` reads `issue.channelId` (used for the "Discussed in" origin row), and issues carry their origin channel as the `h` tag when created with `buzz issues create --channel`. So the association exists end to end; it is only never presented per channel.

### Why it matters

Splitting work across channels is the app's own suggestion — **Add channel** offers "Add another stream to this project". But once work is split that way, the task list stays undivided, so each channel loses its own agenda. For a team that runs one channel per work type, the project-wide list is the only view, and it grows past usefulness quickly.

### Suggestion

Show a task list scoped to the current channel in the channel view, filtered on the origin channel, with an obvious way to jump to the full project list. The filter itself needs no new data.