feat(pgdb): add PG* environment variable fallback in OpenPool

When no DATABASE_URI or config file is found, fall through to
pgxpool.ParseConfig("") which natively reads standard PG* variables
(PGHOST, PGUSER, PGPASSWORD, PGDATABASE, etc.). This removes
unnecessary ceremony in CI and container environments where PG* vars
are already set.

database/pgdb/CLAUDE.md

@@ -97,8 +97,9 @@ For higher connection limits, set via `PoolOptions` or URI query parameter `?poo
 
 ## Environment Variables
 
-- `DATABASE_URI` - PostgreSQL connection URI (takes precedence over config files)
+- `DATABASE_URI` - PostgreSQL connection URI (takes precedence over all other methods)
 - `DATABASE_CONFIG_FILE` - Override config file location
+- Standard `PG*` variables (`PGHOST`, `PGUSER`, `PGPASSWORD`, `PGDATABASE`, `PGPORT`, `PGSSLMODE`, etc.) - used as fallback when no config file is found; parsed natively by pgx
 
 ### URI Format