docs(plans): add monolith S07 repository CRUD plan

What

Adds the implementation plan for monolith/S07 Repository CRUD (Phase 1, hosted), decomposing the approved S07 spec into eight MR-sized, EE-only monolith steps behind the shared artifact_registry_ui flag.

Shape

Two parallel tracks:

• Mutation track: Step 1 (GraphQL foundation — S07-owned base mutation, ArtifactRegistryRepository type + enums, single-repository read) → Steps 2/3/4 (create/update/delete mutations, siblings).
• Vue track: Step 5 (landing + router scaffolding, schema-independent) → Steps 6→7→8 (create → edit → delete flows; the delete button lives on the edit view).

Scope

• Phase 1 (hosted) only. Phase 2 (remote/virtual) is gated on a future Go-service S17 management phase plus an unspecified S02 client extension, so it is deferred to a follow-up plan.

Coordination

• The plan has S07 own Mutations::ArtifactRegistry::BaseMutation per both the S03 and S07 specs. The merged S03 plan over-reaches its own spec by creating it; that is resolved in the S03 plan-amendment at !1089 (merged), which drops the shipped base mutation and keeps criterion-4 coverage via a test-only mutation. This plan's body deliberately carries no callout (93a6d11f, 3859d37a) — once the amendment merges the two plans agree and there is nothing left to flag.

Notes

• Stacked on the open S07 spec branch (zcuddy/monolith-s07-repository-crud); GitLab will retarget to the default branch when the spec MR merges.
• Reviewed once via the plan-authoring review pass; both gap-clearing rounds folded in; Open questions is empty.

Implements monolith/S07.