feat: migrate adapters to sdk

[michaelmalave]

Summary

This branch simplifies the Postgres SDK migration on top of feat/heroku-sdk-integration by removing an intermediate adapter layer and calling SDK APIs directly in PG command handlers. Scope is limited to PG backups/credentials/upgrade command internals and deletion of the now-redundant PG SDK adapter file.

• Refactor PG command implementations to invoke SDK APIs directly in command code
• Remove src/lib/pg/sdk-adapter.ts and its indirection helpers
• Update PG backups, credentials, and upgrade command paths to use the new direct-call pattern

Type of Change

• fix: Bug fix or issue (patch semvar update)
• feat: Introduces a new feature to the codebase (minor semvar update)
• perf: Performance improvement
• docs: Documentation only changes
• tests: Adding missing tests or correcting existing tests
• chore: Code cleanup tasks, dependency updates, or other changes

Verification

git diff --name-only feat/heroku-sdk-integration..HEAD
git log --oneline feat/heroku-sdk-integration..HEAD

# Recommended targeted checks
npm test -- test/unit/commands/pg/backups/index.unit.test.ts
npm test -- test/unit/commands/pg/credentials.unit.test.ts
npm test -- test/unit/commands/pg/upgrade

Additional Context

• Breaking: none expected
• Risk: low to medium; internal PG command refactor with no intended UX changes

Related Issue

Closes #[Github issue number]