fix(claude-code): keep plugin manifest version current

[koriyoshi2041]

Summary

• Bumps the Claude Code plugin manifest from 0.7.1 to 0.8.3 so plugin update checks can see the current shipped version.
• Updates the core release script to bump the Claude Code plugin manifest during v* releases.
• Adds a manifest test so the plugin version cannot fall behind the core release version again.

Tests

• uv run pytest tests/test_manifest.py -q
• uv run pytest tests/ -q
• bash -n scripts/release.sh scripts/release-integration.sh
• git diff --check HEAD~1..HEAD

Note

• The default commit hook did not complete locally because control-plane ESLint could not load @eslint/js in this checkout. The focused Claude Code integration tests and release-script syntax check above passed.

Closes #2386