Add agent-class filter (All / DAP / Connected) to AI Governance dashboard

What

Adds an agent-class filter (All / DAP / Connected) to the AI Governance dashboard Overview, wired to aiGovernanceMetrics(agentClass:), plus a "Showing N agents in the last 7 days" line so the fixed window is explicit. Closes #618950.

Agent-class values

Exposes every value AgentClassEnum currently supports: ALL (All agents), INTERNAL_DAP (DAP), EXTERNAL (Connected). New classes get surfaced here as they're added to the enum. EXTERNAL has no data until external-session ingestion writes agent_type.

Screenshot

Out of scope

Violations/Blocked (no backend) and the time-range switch (#610984) — the dashboard keeps the fixed 7-day window. Filters the summary tiles now; the audit log and ranking cards pick up the filter once their APIs / real data land.

Note

Frontend only, behind the ai_governance_dashboard feature flag.