Release v1.0.3

[sf-jin-ku]

Summary

• bump cc-plugin-codex and the plugin manifest from 1.0.2 to 1.0.3
• add missing changelog coverage for v1.0.2 and the new v1.0.3 release
• add a changelog gate so check, prepack, CI, publish, and npm version all fail when the current version is missing from CHANGELOG.md
• include the intended README refresh and updated image assets, including the new social-preview image

Why

1.0.2 went out without a changelog entry. npm versions are immutable, so the corrective path is to add the missing gate and publish a new version rather than trying to replace the existing 1.0.2 package.

Verification

• npm run check:version-sync
• npm run check:changelog
• node --test tests/changelog.test.mjs
• npm run test
• npm run test:integration
• npm pack --dry-run

Notes

• This PR also backfills the missing v1.0.2 section so the release history stays complete.
• The local Codex CLI available in this environment is older than the plugin-install API used by current E2E scenarios, so the local release verification focused on the repo-level gates, unit tests, integration tests, and pack validation. CI will still run the repository workflow from GitHub's clean environment.