* feat: add provider setup infrastructure
Add the backend commands, config services, shared types, and frontend API layer needed for provider setup and credential management.
* fix: harden provider setup commands and config deletion
Resolve PR review issues by deriving provider behavior from backend definitions, preventing renderer-supplied agent commands, and clearing OAuth cache artifacts when provider config is removed.
* fix: expose provider defs within backend
Restore backend compilation by making provider definitions visible to sibling modules that derive secure config behavior from the shared provider metadata.
* refactor: streamline checkAgentAuth function signature
Simplify the function signature of checkAgentAuth in agentSetup.ts by removing unnecessary line breaks, enhancing code readability.