Build out 10 conceptual guides matching TS SDK

[nficano]

Mirror /Users/nficano/code/arpc/typescript-sdk/docs/guides/:
auth, delegation, errors, job-events, jobs, leases, observability, resume, sessions, vendor-extensions.

Use idiomatic Go (channels, context, errgroup patterns). Place under docs/guides/. Link the spec at spec/docs/draft-arcp-02.1.md.

---

Audit reference: ARCP SDK Audit v1.1 (2026-05-19)