Add an architecture-constraint worked example

[anusbutt]

Context

The current worked example explains a flaky-test correction well, but a second domain would show that GraphKeeper stores durable project knowledge rather than only test results.

Scope

Add a small, generic example in which an agent discovers an architecture or configuration constraint, records exact tool-output evidence, retrieves it, and later supersedes it.

Keep all data fictional and vendor-neutral.

Acceptance criteria

• The example includes valid entities, two closed runs, evidence, and a two-claim supersession chain.
• graphkeeper check and graphkeeper doctor accept the copied fixture.
• The walkthrough shows the active query result and how to inspect exact evidence lines.
• An end-to-end test verifies provenance in both directions.