Stamp schedule-dispatched runs with a $eve.schedule attribution attribute

[warmwind]

Problem

The only durable trace a schedule execution leaves is $eve.trigger = 'schedule' on workflow runs — nothing records which schedule fired:

• run history cannot be aggregated per schedule ("did daily run last night? what's its success rate?" is unanswerable);
• handler-form schedules that dispatch through receive(channel, ...) produce sessions whose $eve.trigger is the channel kind, not schedule, so filtering by trigger misses them entirely — and that dispatch path is a first-class pattern (Handler-form schedule → channel receive() fails in eve dev with LoadCompiledModuleMapError #104);
• observability tooling is left with trigger-based heuristics and cannot attribute runs precisely.

Proposal

ScheduleDispatcher is the single choke point for all schedule dispatch (markdown via runMarkdown; handler-form receive() runs in its context too — see the analysis in #387). Stamp the runs it dispatches with:

• $eve.schedule = the schedule id (optionally $eve.schedule_cron as well);
• sessions spawned in a schedule context carry the attribute, propagated the same way as $eve.root / $eve.parent;
• $eve.trigger semantics stay unchanged — channel-dispatched sessions keep the channel kind; this only adds provenance.

Pure metadata addition, no behavior change, backward compatible. With it, attributes ->> '$eve.schedule' = <id> yields any schedule's complete run history regardless of how the handler dispatched.