Add how-to guide for connecting to PostgreSQL via psql (#188)

## Summary
- Add new how-to guide (`connect-to-postgres.md`) with the `psql` command using `op read` for 1Password credentials
- Add "Database" section to the how-to index linking to the new guide
- Link the new guide from the PostgreSQL reference card's Related section

## Test plan
- [x] Verified `psql` connection works from gilbert using the documented command
- [ ] Review doc formatting and content

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/188

docs/how-to/how-to.md

@@ -38,6 +38,12 @@ Task-oriented instructions for common BlumeOps operations. These guides assume y
 |-------|-------------|
 | [[review-documentation]] | Periodically review and maintain documentation |
 
+## Database
+
+| Guide | Description |
+|-------|-------------|
+| [[connect-to-postgres]] | Connect to PostgreSQL as a superuser via psql |
+
 ## Operations
 
 | Guide | Description |