docs: add concrete agent-failure example to "Why a harness" (#91)

The "harness IS the product" claim reads abstract without a worked
example. Adds a blockquoted, 3-line sidebar inside the "Why a harness"
section showing one realistic failure mode: an agent reaches for a
reverse import (src.models → src.tools), import-linter blocks it in CI
against the "src.models depends on nothing in src/" contract, the
agent's next iteration routes around it via docs/BOUNDARIES.md.

Names a real gate, cites the real contract, links the real doc — so
the example is verifiable, not theatre.

Closes #91

Author: constk

README.md

@@ -98,6 +98,8 @@ analogous to "Hello page" above.
 
 The differentiator isn't the scaffold — it's that every layer of the pipeline catches a different failure class **without relying on the human or LLM coder remembering to run anything**. The same posture protects code regardless of who wrote it.
 
+> **Example.** An agent added `from src.tools import ...` inside `src.models` for type reuse. `lint-imports` failed CI — the `src.models depends on nothing in src/` contract broke — and pointed the next iteration at [`docs/BOUNDARIES.md`](docs/BOUNDARIES.md). The type moved into `src.models` instead. Never shipped.
+
 See [`docs/HARNESS.md`](docs/HARNESS.md) for the full umbrella. Highlights:
 
 - **Pydantic `StrictModel` everywhere a contract crosses a seam** (rejects unknown keys at construction).