docs: establish module versioning boundaries (#4)

Record the accepted rule for root-module versions and nested modules
for heavy dependencies or independent releases. Update repository
instructions and link implementation evidence to the ADR.

Closes #2

AGENTS.md

@@ -6,7 +6,7 @@
 - Documentation: `go run ./cmd/docs check`; create records with `go run ./cmd/docs new adr|plan "Title"`. This checker repository uses its local implementation as the source of truth.
 
 ## Scope and decisions
-- Before editing under a directory with its own AGENTS.md, read it. Scoped files: none; this root governs the single Go module.
+- Before editing under a directory with its own AGENTS.md, read it. Scoped files: none currently; this root governs the repository, including nested Go modules unless nearer instructions override it.
 - Explicit user instructions win; nearer scoped rules override broader rules.
 - Distinguish confirmed requirements, proposed defaults and unresolved decisions; resolve material ambiguity before dependent work.
 - Proposed ADRs are context, not authority; read relevant accepted decisions before changing their scope.
@@ -30,7 +30,7 @@ Go module `go.auvem.com/corestack`.
 
 - Docs format (the spec `docs check` implements): `docs/docs-format.md`.
 - Decisions live in `docs/adr/`, plans in `docs/plans/`. Records are permanent; never rename them.
-- Dependencies are limited to `go.yaml.in/yaml/v3` and `github.com/yuin/goldmark`.
+- Add dependencies only to the module that needs them; heavy dependencies go in nested modules (see the [module-boundary ADR](/docs/adr/20261025T214616275Z-version-light-core-packages-in-the-root-module-and-nest-heavy-dependency-modules.md)).
 
 ## Go
 - Default backend language is Go; follow existing package boundaries.