[codex] Clarify Codex Desktop plugin install

[rob163]

Summary

• Clarify that codex plugin marketplace add registers the marketplace, while installation happens through Codex Desktop's Plugins UI or the CLI /plugins browser.
• Keep codex plugin add ... documented only as an optional shortcut for newer CLI builds that support it.
• Replace the GitHub Pages demo with a project-specific cli-panel.html for this repository's install, validation, preview, and publishing commands.
• Update the README preview image and link text so the README showcases the project command panel instead of the generic template.

Repository metadata

• Updated the GitHub About description.
• Added repository topics: offline-html, codex, claude-code, cli-panel, command-ui, agent-skill, html-effectiveness, agent-skills, skills, developer-tools, static-html, localstorage, workflow.

Validation

• npm run validate
• git diff --check
• Playwright screenshot regenerated for docs/images/cli-panel-preview.png
• Playwright mobile screenshot smoke check for docs/demo/cli-panel.html