#22923: feat(auth): add qwen-agent plugin auth choice support

## Summary - add bundled `qwen-agent-auth` plugin package (device OAuth flow + provider metadata) - wire new `qwen-agent` auth choice into onboarding/auth-choice flows - auto-enable `qwen-agent-auth` when a qwen-agent profile exists - update docs to show qwen-agent as primary Qwen OAuth path while preserving qwen-portal as legacy - add e2e/unit coverage for auth choice routing and plugin auto-enable behavior ## Validation - `pnpm test src/config/plugin-auto-enable.test.ts` - `pnpm vitest run --config vitest.e2e.config.ts src/commands/auth-choice-options.e2e.test.ts src/commands/auth-choice.e2e.test.ts` <!-- greptile_comment --> <h3>Greptile Summary</h3> Added bundled `qwen-agent-auth` plugin package with device OAuth flow and provider metadata, wired the new `qwen-agent` auth choice into onboarding and auth-choice flows, configured auto-enable when a `qwen-agent` profile exists, and updated docs to show `qwen-agent` as the primary Qwen OAuth path while preserving `qwen-portal` as legacy. - Added new plugin under `extensions/qwen-agent-auth/` with device code OAuth flow implementation - Integrated `qwen-agent` auth choice across all auth routing surfaces (auth-choice.apply.ts, auth-choice-options.ts, preferred-provider mapping, onboard types) - Added plugin auto-enable mapping for `qwen-agent-auth` when `qwen-agent` provider is configured - Updated documentation to position `qwen-agent` as primary with `qwen-portal` as legacy alternative - Added comprehensive test coverage (76 new test cases) for auth choice routing and plugin auto-enable behavior <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk - The implementation follows established patterns exactly (mirrors qwen-portal-auth, google-antigravity-auth, and minimax-portal-auth), has comprehensive test coverage with 76 new test cases passing, maintains backward compatibility by preserving qwen-portal as legacy, uses standard OAuth2 device code flow, and updates all necessary integration points (auth routing, plugin auto-enable, docs) consistently - No files require special attention _{Last reviewed commit: 40eb382} <!-- greptile_other_comments_section --> _{(4/5) You can add custom instructions or style guidelines for the agent [here](https://app.greptile.com/review/github)!} <!-- /greptile_comment -->