feat(skill): add PostgreSQL operational skill (#245) (#265)

Add a one-tool PostgreSQL operations skill: configuration review, index and
query-plan analysis, vacuum/bloat, WAL archiving + point-in-time recovery,
replication/failover, extensions, upgrades, and evidence-based diagnostics.
Ships the read-only pgdiag collector (stdlib, --json, --plan-for, --help
without a cluster), 9 dated references, tests, a human README, 6 eval cases,
and the top-level index + regenerated catalogs. Routes app data access to
backend-engineering and schema design to data-architect/data-engineering.

Closes #245

Co-authored-by: factory-droid[bot] <138933559+factory-droid[bot]@users.noreply.github.com>

.codex-plugin/plugin.json

@@ -98,6 +98,7 @@
     "./peertube",
     "./platform-engineering",
     "./playwright",
+    "./postgres",
     "./privacy-engineering",
     "./product-adoption",
     "./product-analytics-and-measurement",