Add release consistency checks for plugin version and docs

[baskduf]

Description

Release preparation currently requires keeping several version references in sync manually:

• plugins/codex-fable5/.codex-plugin/plugin.json version
• CHANGELOG.md latest release section
• README*.md install refs
• the eventual git tag vX.Y.Z

Add an automated consistency check so release PRs fail early when these drift.

Acceptance Criteria

• Add a script or unittest that reads the plugin manifest version.
• Assert CHANGELOG.md contains a matching ## <version> - section.
• Assert all README install examples reference v<version>.
• Assert stale previous release refs do not remain in README*.md install commands.
• Include the check in the existing CI/unittest flow.
• Provide clear failure messages that name the mismatched file/value.

Notes

This should reduce the chance of future release PRs shipping mismatched docs or plugin metadata.