Build out 10 conceptual guides matching TS SDK

[nficano]

F# SDK has README + samples, no conceptual docs. Mirror the TS SDK guide set at /Users/nficano/code/arpc/typescript-sdk/docs/guides/:

• auth.md, delegation.md, errors.md, job-events.md, jobs.md, leases.md, observability.md, resume.md, sessions.md, vendor-extensions.md

For each guide:

1. Read the TS version for structure and depth
2. Write an F#-idiomatic equivalent in docs/guides/ of this repo
3. Use F# code samples, F#-native types/records, and computation-expression patterns where natural
4. Link to the relevant spec section (spec/docs/draft-arcp-02.1.md)

Add docs/README.md index pointing to the guides.

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