feat(cli): improve info command with connection status and add introspect usage examples (#17)

* feat(cli): improve info command with connection status and add introspect usage examples

- Display connection status in info command output
  - Show "(from actual connection)" when connected to database
  - Show "(default value - no database connection)" when not connected
  - Use actual dialect from backend instead of creating new instance
  - Display installed extensions when connected
- Add detailed usage examples to introspect subcommand help
  - Examples for all introspection types (tables, views, columns, etc.)
  - JSON output example
  - Schema parameter example (PostgreSQL specific)
  - Environment variables connection example

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* chore: add changelog fragment for PR #17

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: vistart

changelog.d/17.added.md

@@ -0,0 +1 @@
+Improved PostgreSQL backend CLI info command with connection status display and added comprehensive introspect usage examples.