Add Sandbox lifecycle and Runtime activity metrics

[sam2tom]

Summary

• expose provider-neutral managed Runtime lifecycle state in the public observation contract and strict client projection
• show current Sandbox active, sleeping, transitioning/pending, and total counts on Dashboard
• replace Compute uptime charts with Session-level Runtime active 0/1 stepped series for live and durable views
• aggregate durable activity across allocations and project missing or unavailable buckets as inactive
• preserve the latest main branch Session metrics cache and no-flicker tab behavior

Validation

• make openapi
• Core Web: 70 files / 690 tests passed; typecheck and production build passed
• agents-client: 9 files / 380 tests passed; typecheck passed
• focused Runtime Observation Go tests passed
• Agents API contract Go tests passed
• two affected Playwright Dashboard acceptance tests passed
• independent blind review completed with no remaining blockers or material nonblockers
• make check was started and passed its console, installer, manifest, shell, and console-build stages, then stopped because PARSAR_AGENTS_API_TEST_DATABASE_URL is not configured on this host

Scope

• no database migration or ClickHouse dependency
• no provider-native identifiers exposed
• no lifecycle mutations
• historical sleeping versus collection failure is intentionally not distinguished yet; both project to 0