Fix npm README image link

[fouad-openai]

Motivation

• The image referenced in the package README was 404ing on the npm package page because it used a relative file path that doesn't resolve on npm, so the splash image needs a GitHub-hosted URL to render correctly.

Description

• Update README.md to replace the relative image path ./.github/codex-cli-splash.png with the GitHub-hosted URL https://github.com/openai/codex/blob/main/.github/codex-cli-splash.png.

Testing

• No automated tests were run because this is a docs-only change and does not affect code or test behavior.

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Codex Task