Clarify agent simplicity guidance

#### Context

Agent guidance did not clearly state that simplicity is a project
constraint or that adversarial review should challenge complexity.

#### TL;DR

*Make agent guidance prefer simple designs and adversarial verification.*

#### Summary

- Add one-owner/invariant and lifecycle review guidance.
- Prefer narrow real OTP tests and stable observable proof.
- Ask agents to challenge complexity and push back early.

#### Alternatives

- A detailed Symphony-specific checklist was rejected because it would
  overfit one incident instead of providing durable guidance.

#### Test Plan

- [x] `cd elixir && mise exec -- make all`