test: cover PKCE callback token exchange

[WilsonLe]

Closes #63

Summary

• Add mocked provider regression coverage for PKCE callback token exchange.
• Assert the callback request forwards the authorize-issued pkce_verifier cookie into the default token request as code_verifier.
• Move agent instruction files from .github/instructions to docs/instructions and keep PKCE callback coverage guidance there.

Credit

Thanks to @arcassia for reporting #63 and opening #64 with the original pkceEnabled / req callback fix.

Testing

• pnpm install --frozen-lockfile
• pnpm test
• pnpm build
• pnpm exec prettier --check docs/instructions/*.instructions.md test/mocked-provider-integration.spec.ts