Name the file and heading wherever a rule cites another section #2134

Part of #2124.

An agent resolving a reference has to search the tree for it.

• A section ID in prose, such as quality-cross-validation (core/quality-gates.md:29, :107) or testing-negative-assertion-coverage (:104)
• base-<language>-tooling (core/quality.md:54, :97) names a template upstream no longer ships
• backend/edge.md:28 cites observability.md by bare file name

Done when

• Every cited ID carries its file and heading beside it
• The tooling reference is qualified for a project without one