Release v2.0.0

Release bookkeeping only: VERSION to v2.0.0, the [Unreleased] section headed ## [2.0.0] -- 2026-09-03, and just sync propagating the version into the five .reference.yaml pins so newly stamped projects start on the current schema.

Contents: the exemption mechanism reduced to standard plus reason, with the time basis, applicable_standards and adoption_phase removed (a MAJOR change); enforcement claims matched to the code; the start surface collapsed to one path and one first move; the unreferenced design template and the uncalled check_toml_key primitive removed; the groundskeeper doctrine page and the agent-surface validator. The catalog pin stays at v5.0.0. The tag lands on the merge commit, as v1.4.0's did, with the [2.0.0] section as its body.