* docs: rewrite context-as-code as reviewing-context guide
Move the page from Concepts to Guides and rebuild around an interactive
review-loop diagram. Extract pan/zoom + fit-view controls into a shared
FlowCanvas wrapper and adopt it across all three docs diagrams.
* test: point examples-docs assertion at reviewing-context
Update the doc smoke test that read context-as-code.mdx to read the new
guides/reviewing-context.mdx path. The `ktx ingest --all --no-input`
assertion still holds; the rename was the only break.
