docs: make the Codex manual install path explicit

What I noticed

README.md says the setup prompt works with Claude Code, Codex, Hermes, Openclaw, or any shell-capable agent. The automatic install.md also has a dedicated Codex registration step:

mkdir -p "${CODEX_HOME:-$HOME/.codex}/skills"
ln -sfn ~/Developer/video-use "${CODEX_HOME:-$HOME/.codex}/skills/video-use"

In the README manual install section, the Codex symlink line is present but commented out:

ln -sfn ~/Developer/video-use ~/.claude/skills/video-use        # Claude Code
# ln -sfn ~/Developer/video-use ~/.codex/skills/video-use       # Codex

Suggested docs change

Show Codex as its own explicit manual install option, matching install.md, for example:

mkdir -p "${CODEX_HOME:-$HOME/.codex}/skills"
ln -sfn ~/Developer/video-use "${CODEX_HOME:-$HOME/.codex}/skills/video-use"

Why

Codex users can miss the commented line or copy the Claude-only path. Making both supported agents explicit keeps the manual path consistent with the setup prompt and installer guide.