Stale background subagents persist in UI after session restart/compaction #27373

Description

Background subagents (e.g. librarian, explore) that were launched in a previous session context (before compaction or restart) persist in the UI's "Subagentes" panel as running tasks indefinitely. They cannot be cleared.

Steps to Reproduce

1. Start a session and launch background subagents (e.g. task(subagent_type="librarian", run_in_background=true, ...))
2. Session gets compacted (context reset) or OpenCode is restarted
3. The subagent entries remain visible in the UI sidebar showing "running" state
4. background_cancel(all=true) returns "No running or pending background tasks to cancel"
5. Restarting OpenCode does not clear them

Expected Behavior

• Stale subagent entries should be cleared on session restart or compaction
• Or: provide a CLI command like opencode session clear-tasks <sessionID> to manually clear them
• Or: the UI should detect orphaned tasks and remove/mark them as stale

Actual Behavior

The UI shows ghost entries forever:

▼ Subagentes
  [ ] Research regional MIGs (librarian) ↳ 23:36:01
  [ ] Find GCE aggregated machineType… (librarian) ↳ 23:38:48

These are not actually running — they're leftover references from a pre-compaction context that no longer exists.

Environment

• OpenCode version: 0.6.1
• OS: Linux