integrations: add portable support and project-provider descriptors

[EmersonBraun]

Outcome

Add portable public integration descriptors for Zendesk, Freshdesk, Trello, ClickUp, monday.com, and GitLab.

Acceptance criteria

• listIntegrations() returns each stable provider id.
• Every descriptor exposes public auth metadata, docs, categories, least-privilege scopes, and at least one safe read action.
• A workflow-useful write action exists only where the public provider supports it safely.
• Mocked tests use synthetic credentials and responses.
• Failures are typed and actionable.

Privacy boundary

The descriptors are independently specified from public provider documentation. No private consumer credential path, setup state, runtime behavior, fixture, policy, or implementation detail may be referenced or encoded.