release: unlock v2.0.0 GA after RC gates

[codeforester]

Summary

• promote the verified v2.0.0-rc.1 release line to the v2.0.0 GA candidate
• unlock the repository release guard after the recorded RC consumer and rollback gates
• align VERSION, README, changelog, runtime metadata, and release-policy tests

Issue

Related to #215

Validation

• `git diff --check`
• `bats tests/release.bats` (18 passed)
• `BASE_CACHE_DIR=/private/tmp/base-bash-ga-dry-run scripts/release publish --version 2.0.0 --dry-run --manifest base_manifest.yaml` (requires a clean merged checkout and authenticated hosted context)

Docs Impact

The README and versioning policy now describe v2.0.0 as the stable target after
the verified RC. First-party consumer and Homebrew cutover remain a separate
post-asset handoff.