docs: fluent query API guide (chain algebra, explain, backends)

[rahlk]

PROBLEM

The fluent query API is a new user-facing surface with no documentation. Users need a guide covering the chain algebra, terminators, .explain(), backend behavior, and the Opaque split.

SCOPE BOUNDARY

Docs only. The API reference itself regenerates via the existing sdk-release dispatch; this issue is the narrative guide.

GOALS (the contract, as a checklist)

1. "Querying with the fluent API" guide: entry points, chaining, terminators, explain output walkthrough (using the two PoE audit queries as worked examples).
2. Backend notes: in-memory vs Neo4j, capability degrade labels, strict mode.
3. Migration note for cldk.graph deprecation (once the slicer child lands).
4. Both docs fronts covered (mkdocs main + astro) per repo-map convention.

DEFINITION OF DONE

• mkdocs build --strict and the astro build green with the new pages.
• Worked examples execute verbatim against a released version.

Part of codellm-devkit/python-sdk#279