Dander 0.4: discover installed connector plugins

[harrisonoconnorhover]

What changed

• add presentation-safe GET /v1/connectors discovery to the loopback graph service
• load only exact manifest-declared plugin packages and expose canonical graph bindings
• retain the built-in Salesforce adapter as the deprecated 0.4 fallback
• prepare Dander 0.4.0rc2 because discovery is functional runtime behavior after rc1

The response deliberately omits API URLs, authentication settings, secret references, request
bodies, and credentials. Existing projects without plugins receive an empty dynamic catalog.

Validation

• Ruff lint and format
• strict mypy
• 691 pytest tests
• Terraform format/init-without-backend/validate for platform and stage zero
• dependency audit: no known vulnerabilities
• wheel and sdist validation
• Linux container build and dander 0.4.0rc2 startup

No package publication, Terraform apply, GCP mutation, or retained-project change occurred.