Fix dashboard metric names to match exporter output

[mvillmow]

Grafana dashboards reference metrics the exporter does not produce:

• agent-health.json: uses maestro_agents_active but exporter emits maestro_agents_online
• nats-events.json: uses gnatsd_varz_* but exporter emits nats_in_msgs_total etc.
• task-throughput.json: uses rate() on gauges (maestro_tasks_total) which won't work, references maestro_keystone_dispatch_duration_seconds_bucket histogram that nothing produces

[mvillmow]

Closed: ai-maestro replaced by ProjectAgamemnon per ADR-006 in Odysseus. Migration PRs: Myrmidons#59, ProjectHermes#58, ProjectArgus#58, ProjectTelemachy#64, ProjectScylla#1737, AchaeanFleet#62, ProjectProteus#45, ProjectKeystone#159.