OpenAPI: openapi-client-gradle + api-contract-checks + personal-stack adopt

[jorisjonkers-dev-agents]

Objective: split external client generation (Gradle plugin) from internal contract-drift checks (scripts/CLI).

Work:

• openapi-client-gradle: external spec/client generation conventions
• api-contract-checks: assistant/knowledge export tasks + openapi-typescript contract check + contract-banner + contract-validate logic
• personal-stack updates export tasks, assistant-ui scripts, package.json, contract-validate

Part of #605.