feat(core): run session schedules as global background jobs

Moves schedule timers out of the per-Location plugin into a new global
SessionSchedule service. Previously timers lived in the Location's plugin
scope, so they stopped on server restart until something reloaded the
Location, and stopped whenever the Location was evicted after 60 minutes
of inactivity. Timers now live in the global service and call
Session.prompt, which loads the Location on demand, so schedules fire
even when the session is not open.

- SessionSchedule persists entries in KV (session.schedule/<id>) and
  exposes set/remove/resume. A deleted session ends its schedule.
- The managed server calls resume at boot next to restart continuity;
  unmanaged servers sharing the database do not restore schedules.
- The opencode.schedule plugin now only registers /schedule and
  delegates to the service; the per-Location restore logic is removed.

Verified: core and server typecheck; against an isolated managed-mode
server, a 5s schedule fired every 5s, resumed after a restart with no
client touching the session, and `/schedule off` stopped the
boot-restored timer. Server tests: 3 OpenAI OAuth port tests fail
identically without this change.