[codex] Update README and sync alchemy-cli skill

[0xmts]

Summary

• reorder the skills section in the root README to surface alchemy-cli first and move the alchemy-codex chooser guidance to the bottom of the table
• make CLI installation the primary install path, keep the npx skills add path as the secondary option, and add the official npm link to the links section
• remove Codex plugin maintenance details from the public README and keep the remaining Codex-router guidance with the alchemy-codex skill files
• sync skills/alchemy-cli/SKILL.md from the published upstream repo state

Why

The root README had accumulated internal Codex plugin packaging details that are not useful for external readers. This change makes the public docs CLI-first, keeps the README focused on user-facing install and skill-selection guidance, and moves the small amount of relevant Codex-router context to the router skill itself.

Impact

Users landing on the repo now see the CLI-first path first, the skill selection guidance is cleaner, and the published alchemy-cli skill is present on this branch so the README links resolve correctly.

Validation

• inspected the staged diff for the README and skill files
• confirmed skills/alchemy-cli/SKILL.md matches origin/main byte-for-byte
• ran git diff --cached --check
• verified the commit is signed with the configured SSH signing key

[0xmts]

Superseded by #16 on branch mc/codex/readme-cleanup.