feat(I001): add pg_stat_io metrics support for PostgreSQL 16+ I/O monitoring

Summary

• Add pg_stat_io metric collection for PostgreSQL 16+ I/O monitoring
• Add I001 report generator with buffer hit ratio analysis
• Add Grafana dashboard with 12 I/O statistics panels
• Add express checkup support for single-snapshot analysis

Closes #117 (closed)

Test plan

• Python reporter I001 unit tests cover PG16 happy path, no Prometheus data, and PG18 byte metrics
• Bun checkup tests cover getIOStatistics and generateI001 edge cases
• Config consistency test passes
• JSON/YAML parse checks pass
• Verify metrics collection on PG16+ instance
• Verify graceful degradation on PG < 16