refactor(openapi): Use the shared specification generator

[nfebe]

FlatRun maintains its own OpenAPI generator even though the shared generator now handles route and type discovery. Move generation to github.com/whilesmartgo/openapi so generator fixes live in one package. Preserve the current agent contract, including its version, permission and plan extensions, schema display hints, and route exclusions.

Checklist

• Add reusable options to the shared package for the agent's package discovery, version, extensions, schema hints, and route exclusions
• Generate the agent specification with a pinned release of the shared package
• Keep the drift test running through the same generation command
• Remove the copied generator after the generated contract matches

Related

• https://github.com/whilesmartgo/openapi
• https://github.com/whilesmartgo/openapi/releases/tag/v0.1.0