Build out README and 10 conceptual guides

[nficano]

README is currently minimal; conformance.md is 2 lines. Build out documentation to match TS SDK depth.

Steps:

1. Rewrite README.md with: badge linking to draft-arcp-02.1.md, install snippet, quickstart, package layout, link to docs
2. Create docs/guides/{auth,delegation,errors,job-events,jobs,leases,observability,resume,sessions,vendor-extensions}.md (mirror typescript-sdk/docs/guides/)
3. Create docs/architecture.md, docs/conformance.md (proper version with section-by-section coverage), docs/getting-started.md, docs/transports.md, docs/troubleshooting.md

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