Define module consumption policy for curated downstream modules

[CMCDragonkai]

Problem

nixpkgs-matrix exports module surfaces, but module-consumption boundaries are not yet clearly defined for downstream-curated flakes.

Open questions captured in plans/modules-consumption.md remain unresolved:

• whether this repository should lift downstream nixosModules and homeModules at all,
• how to keep module exposure coherent with curated package distribution,
• what canonical downstream consumption path should be.

Intent

Establish one explicit policy for module consumption/export so package curation and module curation do not drift independently.

Scope

• Decide whether downstream modules are in or out of this repo contract by default.
• If in-scope, define qualification criteria for lifting modules.
• Define downstream consumption guidance for nixosModules and homeModules.
• Define whether additional checks are required for module-surface invariants.

Non-goals

• No private-repo runtime orchestration design.
• No expansion of module catalogs in this issue.

Tasks

1. Write module-consumption policy decision doc.
2. Add README module-consumption guidance aligned to the decision.
3. Add/adjust checks if the decision changes contract behavior.

Acceptance criteria

• README guidance and exported surfaces are consistent.
• Any new/changed invariants are check-backed.

[linear]

INF-150