Bump Decodex plugin version to 0.2.0

[yvette-carlisle]

Summary

• bump the packaged Decodex Codex plugin manifest to 0.2.0
• align the Decodex app staging fallback version with the 0.2.0 release

Validation

• jq . plugins/decodex/.codex-plugin/plugin.json
• cargo test -p decodex plugin_surface_tests::packaged_skills_preserve_research_promotion_and_queue_boundaries --all-features -- --test-threads=1
• plugin-eval analyze plugins/decodex --format json (91/B, no fail; only aggregate token-budget warnings)
• apps/decodex-app/script/build_and_run.sh stage
• staged app Info.plist CFBundleShortVersionString=0.2.0 and CFBundleVersion=0.2.0
• cargo make test-decodex-app-stage
• cargo make fmt
• git diff --check HEAD~1..HEAD

Landing: use Decodex/admin merge only; do not use merge queue.