docs(agent): specify reliable job lifecycle

[samiriasbeck]

Summary

This draft specifies a fail-closed reliability contract for the existing Buzz
agent job kinds (43001 through 43006). It covers:

• bounded, same-channel context packs;
• authenticated attachment materialization with digest/size verification and
  explicit text/image inspection status;
• monotonic lifecycle receipts with terminal latency and failure reasons;
• exact two-agent debate semantics where N cycles require 2N correlated turns;
• per-channel authorization for context, attachments, participants, and results.

No runtime behavior or event kind is changed by this documentation-only draft.

Why a draft

The proposal touches several active design areas. Opening it as a draft gives
maintainers one concrete, reviewable contract while implementation work can stay
split into focused follow-up changes.

Related work

• Relates to Configurable orchestration workflows (user-defined; agent + human-in-the-loop; progress + artifacts) #3871
• Relates to [Feature Request] Agents cannot view image attachments (Blossom media requires NIP-98 HTTP auth) #2770
• Relates to feat: multiparty / per-participant read receipts for messages #3000
• Complements fix(media): enforce channel ACL on bound blobs (follow-up to 769ac70) #5185 by requiring channel-scoped media grants and preventing hash
  reuse from widening access
• Builds on NIP-AO telemetry without treating ephemeral frames as durable job
  completion evidence

Validation

• git diff --check
• DCO sign-off present
• Manual privacy scan: no private relay URLs, local paths, credentials, customer
  names, or deployment-specific identifiers