fix(pgpm/cli): use database name instead of UUID for default extension name

[pyramation]

Summary

Changes the default value for extensionName in the pgpm export command from a UUID to the human-readable database name.

Previously, the default was dbInfo.database_ids[0] which is a UUID like 7c944bc6-511a-44ad-a407-ce2cefd5e466. Now it uses the selected database name from collections_public.database, falling back to the PostgreSQL database name (dbname) if unavailable.

Review & Testing Checklist for Human

• Run pgpm export end-to-end - Verify the extension name prompt now shows a readable database name as the default instead of a UUID
• Verify the fallback works - If selectedDatabaseNames[0] is somehow undefined, confirm it falls back to dbname correctly

Test Plan

1. Run pgpm export against a database with entries in collections_public.database
2. When prompted for "Extension name", verify the default value shown is the database name (not a UUID)
3. Accept the default and confirm the export completes successfully

Notes

Link to Devin run: https://app.devin.ai/sessions/7530c5fa0b95477e9758c657dff7e299
Requested by: Dan Lynch (@pyramation)

[devin-ai-integration]

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