P2-02 (agent side) + P2-03: agent scheduler + schedule.fire dispatch

Closes the schedule reconciliation loop end-to-end.

* New `internal/agent/scheduler` package wraps robfig/cron/v3 with
  the lifecycle the agent needs:
  - Apply(ScheduleSetPayload, Sender) stops the prior cron (waiting
    for in-flight entries to return), rebuilds from scratch, starts,
    and emits schedule.ack with the version we just applied.
  - Disabled entries skipped silently; bad cron exprs (which
    shouldn't reach us — the server validates — but defensive)
    log a warn and skip.
  - On each cron tick the entry sends a new schedule.fire envelope
    to the server with {schedule_id, scheduled_at}. The scheduler
    itself never builds CommandRunPayloads — server is the source
    of truth for jobs.
  - tx is swapped on every Apply, so reconnect is handled
    naturally: cron entries that fire against a dropped tx log
    "no active connection" and skip the tick.
  - Stop() is idempotent and waits for the cron's in-flight
    workers via cron.Stop().Done().

* New wire message api.MsgScheduleFire + api.ScheduleFirePayload
  for the agent → server "I just fired locally" RPC.

* Server-side dispatch (schedule_push.go: dispatchScheduledJob):
  looks up the schedule by id, validates ownership + that it's
  enabled, builds args from kind (paths for backup; other kinds
  are still arg-less in Phase 2 and grow as those job kinds land
  in P2-05..08), persists a jobs row with actor_kind=schedule +
  scheduled_id, and writes command.run back on the same conn so
  the agent runs through its existing dispatch path.

* store.CreateJob now writes scheduled_id. This column was in the
  schema since 0001 but never populated — the original P1 path
  only had operator-driven jobs, so actor_kind was always 'user'
  and scheduled_id was always nil.

* cmd/agent/main.go integration: dispatcher gains a
  *scheduler.Scheduler; the MsgScheduleSet case now hands the
  payload to scheduler.Apply (in a goroutine so the WS read loop
  keeps draining other messages).

* WS dispatcher gains OnScheduleFire alongside OnScheduleAck.

* Tests:
  - scheduler unit tests (4): ack-on-apply, cron tick fires
    schedule.fire envelope, disabled entries don't fire, replace-
    prior-state stops the old cron.
  - Server-side end-to-end: schedule.fire → command.run with the
    right job_id / kind / args, plus jobs row with actor_kind=
    "schedule" and scheduled_id linking back to the schedule.

Persistence of next-fire times across agent restarts is
deliberately deferred. A missed fire window during downtime
simply fires once on reconnect — that's the desirable behaviour
(the operator wants the missed backup to run, not be silently
skipped because we lost track of when it was due).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/api/messages.go

@@ -197,6 +197,17 @@ type ScheduleAckPayload struct {
 	AppliedAt  time.Time `json:"applied_at"`
 }
 
+// ScheduleFirePayload — agent reports a local cron entry just fired.
+// Server is expected to look up the schedule, build a CommandRun
+// payload from it, persist a job row, and return MsgCommandRun on
+// the same connection. ScheduledAt is the wall-clock time the
+// agent's cron fired (audit / forensic value when network jitter
+// pushes the actual command.run dispatch later).
+type ScheduleFirePayload struct {
+	ScheduleID  string    `json:"schedule_id"`
+	ScheduledAt time.Time `json:"scheduled_at"`
+}
+
 // ConfigUpdatePayload — server pushes per-host config (currently just
 // repo connection details). Empty fields mean "leave existing alone";
 // to clear something, send an explicit zero value.

internal/api/wire.go

@@ -21,6 +21,7 @@ const (
 	MsgRepoStats      MessageType = "repo.stats"
 	MsgLogStream      MessageType = "log.stream"
 	MsgScheduleAck    MessageType = "schedule.ack"
+	MsgScheduleFire   MessageType = "schedule.fire"  // agent: a local cron entry fired, please dispatch a job
 	MsgCommandResult  MessageType = "command.result"  // ack for command.run
 	MsgError          MessageType = "error"
 )