feat(core): add /schedule command for recurring session prompts

Adds a minimal built-in plugin (opencode.schedule) that registers
`/schedule <30s|15m|2h> <prompt>` and `/schedule off`. Each schedule is
saved in plugin storage keyed by session ID and re-queues the prompt with
delivery "queue" on a fixed interval, modeled on the warming plugin loop.
When a location loads, the plugin restarts only schedules whose session
belongs to that location, so a session is never double-scheduled.

Known gaps (prototype): schedules resume after a server restart only once
something loads the session's location, and intervals longer than the
60-minute location inactivity TTL stop when the location is evicted.

Verified: core typecheck passes; against an isolated `serve` instance, a
5s schedule admitted prompts every 5s, stopped across a restart, and
resumed after the session's location was reloaded by a manual prompt.