Fix leaked active leases and restore lease summary fields

[krandder]

Summary

• reject LeaseGranted events with empty agent ids and reject blank claim agent ids at the HTTP edge
• have the core clock recover malformed active tasks with missing lease metadata or blank active agents instead of leaving them stuck in active
• restore activeAgent, leaseExpiresAt, and updatedAtMs on /tasks summaries and add a recovery script for existing leaked leases

Verification

• npx tsx --test core/test/validator.test.ts core/test/scenarios.test.ts
• npm run test:middle
• node --import tsx scripts/recover-leaked-leases.ts --dry-run