feat(connectors): extend ConnectorDefinition with provider/auth/onboarding

- Add PiOAuthProviderId literal type covering pi-ai's built-in OAuth
  providers (anthropic, openai-codex, github-copilot, gemini-cli,
  antigravity)
- Add optional ConnectorDefinition fields: provider (pi-ai provider id),
  auth (ConnectorAuth with kind/type/provider/oauthId/envKey), and
  onboardingFeatured
- Decorate built-in CONNECTORS (codex_local, claude_local, gemini_local,
  claude-compat slots) with their pi-ai provider id, auth schema, and
  onboarding flag so credential-resolver and oauth-login-service can
  dispatch without changing execute/module wiring
- Re-export CONNECTORS, findConnectorDefinition, and
  listFeaturedOnboardingConnectors from main/connectors so consumers
  (including tests) import from a single module

src/main/connectors.ts

@@ -1,6 +1,8 @@
 import type { ConnectorRecord } from "@shared/types";
 import { CONNECTORS } from "./connector-definitions";
+export { CONNECTORS } from "./connector-definitions";
 export type { ConnectorDefinition } from "./connector-definitions";
+export { findConnectorDefinition, listFeaturedOnboardingConnectors } from "./connector-definitions-lookup";
 
 const RESERVED_ENV_BINDING_KEYS = new Set([
   "PATH",