docs: add a concrete agent-failure example to make the harness claim tangible

[constk]

Problem

CLAUDE.md and the harness docs assert that the gates catch agent mistakes, but never show one. The claim "harness IS the product" reads abstract without a worked example. New readers (human or agent) have to take the promise on faith.

Proposed solution

Add a 3–5 line sidebar / callout to either CLAUDE.md or README.md with a concrete, real-feeling example. Suggested wording:

Example: an agent added a reverse import (tools → models). The lint-imports gate failed CI and pointed the agent at docs/BOUNDARIES.md. The mistake never shipped.

If a real captured incident exists (PR/CI run), link it instead of the synthetic example.

Acceptance criteria

• One concrete example documented in CLAUDE.md or README.md
• Example names a specific gate and the doc it routes the agent to
• Stays under ~5 lines to avoid bloating either file

Priority rationale

Cheapest way to convert an abstract claim into something a reader can picture. Pairs naturally with the README opener reframe.

[constk]

Closed via #100 (merged to develop as commit 7c84f18). Awaiting the next release: PR to reach main.