Rewrite inline comments added during the agent identity hardening so
they explain the code as it stands today, rather than tying to internal
plan terminology (phase numbers, FR/NFR/H/M/C codes, PR references,
trap zone labels). Commit history already carries the plan archaeology.
Comments now keep the non-obvious invariants (cache boundaries, bypass
gaps, silent-nil traps, dual-tenant semantics) and drop the scaffolding.
Comment-only — no runtime behavior change.
Publish-time observer warns (does not block) when DomainEvent.AgentID is
non-empty and fails uuid.Parse. Distinct log field `non_uuid_agent_id`
avoids colliding with existing telemetry that parses `agent_id` as UUID.
Safety net for PR #826-class regressions. Validator lives in bus.Publish
so all call sites are covered without per-publisher instrumentation.
Phase 1 Fix C of agent identity hardening.