Agents window: built-in tools for remote session delegation

Add built-in tools for delegating work across connected remote agent hosts from the Agents window, without host/workspace pickers.

- Discover hosts and select by platform, CPU/memory, model availability, and current workload.
- Create workspace-less sessions or use an existing trusted target folder/worktree.
- Send follow-ups and replies to the exact originating chat, and inspect remote status and the latest response.

The MVP retains normal approvals and requires the coordinating Agents window to stay connected. Repository cloning/source transfer and operation-budget redesign are separate follow-ups.