[FEATURE]: /schedule — run recurring background tasks (cron-like routines)

Requested feature

Add a /schedule slash command (inspired by Claude Code's /schedule) to run recurring, background, non-interactive tasks on a timer — e.g. periodic CI/CD sweeps, lint/typecheck runs, or repo reviews.

Why

Today opencode has an internal BackgroundJob service (packages/core/src/background-job.ts, packages/opencode/src/background/job.ts) used for things like session compaction and summary pruning, but there is no user-facing way to schedule a recurring job from the TUI and no cron-like cadence. Automations currently require external orchestrators (n8n, scripts).

Proposed behavior

• /schedule every 5m <task> or /schedule cron '*/5 * * * *' <task> registers a recurring background routine.
• Jobs appear in a scheduler list (pause/resume/cancel), run detached from the active session, and surface results via the existing notifications/background job UI.
• Reuse/extend BackgroundJob.Service (packages/opencode/src/background/job.ts) with a persistence + scheduling layer so jobs survive across sessions.

Benefits

• In-terminal recurring automation without external tooling.
• Consistent with the existing background job infrastructure.