[FEATURE]: /tasks — list and manage background subagents running in parallel

Requested feature

Add a /tasks slash command (inspired by Claude Code's /tasks) to list subtasks/subagents running in parallel in the background.

Why

openCode already has the machinery: BackgroundJob.Service (packages/opencode/src/background/job.ts), the experimental background-subagent endpoint (packages/opencode/src/server/routes/instance/httpapi/handlers/experimental.ts, /experimental/session/:id/background), and background.promote / background.cancel. But these are only surfaced via internal/experimental APIs — there is no user-facing /tasks view.

Proposed behavior

• /tasks shows active background jobs/subagents with status (running/completed/error/cancelled), plus promote/cancel actions wired to BackgroundJob.Service.
• Build on the experimental background-subagents flag to promote/join blocking subagents into the background from the TUI.

Benefits

• Visibility into parallel work without digging into debug endpoints.
• Promotes the existing background-job infrastructure to a first-class UX.