Implement single-entity action monitors (audit log/reporter/Prometheus)

SingleEntityActionMonitor is a pure ABC with no concrete implementations, and no service wiring passes monitors to the single-entity processor (defaults to None). Actions already migrated to the single-entity framework therefore lose the observation that the legacy BaseAction framework provides via actions/monitors/ (AuditLogMonitor, ReporterMonitor, PrometheusMonitor).

Scope:

Implement the audit-log, reporter, and Prometheus monitors for the single-entity action framework, consuming the single-entity contract (BA-7054 already preserved operation_type()/spec() on the pure-ABC base for the audit-log/reporter keys).

Wire the monitors into the single-entity processor via the DI composer so migrated actions are audit-logged, reported, and metered again.

Decide per-type duplication vs. a thin shared implementation consuming only common fields (epic open question 1).

JIRA Issue: BA-7059

Start with the SingleEntityActionMonitor ABC, the existing actions/monitors implementations (AuditLogMonitor, ReporterMonitor, and PrometheusMonitor), and the single-entity processor's DI composer. Compare the legacy monitor contracts with the single-entity operation_type()/spec() contract, then verify that migrated actions are audit-logged, reported, and metered through the processor.

Written by the indexing model from the issue text.