fix(codex): drop dead skill stub and lowercase plugin branding (#1223)

The Codex plugin shipped a placeholder skill (plugins/codex/skills/codemem/SKILL.md) that only described the plugin; it provided no behavior and has no equivalent in the Claude or OpenCode integrations. Remove it and its plugin.json `skills` reference.

Also lowercase user-facing branding to `codemem` (interface.displayName and defaultPrompt in the Codex plugin manifest, and displayName in the Codex marketplace manifest) to match the project's preferred stylization.