fix: correct plugin install command + add Codex install docs

[oceans404]

Summary

• Fix the Claude Code install command, which was failing because the marketplace name in .claude-plugin/marketplace.json is stellar-dev, not stellar-dev-skill. Updates README and the site installer card to /plugin install stellar-dev@stellar-dev.
• Add an OpenAI Codex install section (clone into ~/.codex/skills/) to both the README and the landing-page installer cards.

Repro of the install bug (closes #28)

> /plugin marketplace add stellar/stellar-dev-skill
  └ Successfully added marketplace: stellar-dev

> /plugin install stellar-dev@stellar-dev-skill
  └ Marketplace "stellar-dev-skill" not found

Reported by @iamjayrome on Twitter: https://x.com/iamjayrome/status/2057528609599111530

Codex docs (part of #27)

Addresses one item from @jamesbachini's feedback in #27: surface a one-line Codex install command. Path uses ~/.codex/skills/ to match the existing Skill Directory table in the README.

Test plan

• Run /plugin marketplace add stellar/stellar-dev-skill then /plugin install stellar-dev@stellar-dev in Claude Code and confirm install succeeds.
• Build the site and confirm the Installing card grid shows both the corrected Claude Code command and the new Codex card.
• Confirm ~/.codex/skills/stellar-dev-skill clone is the right pattern for Codex skill discovery (or adjust path).

[Copilot]

Pull request overview

This PR fixes the documented Claude Code plugin install command to reference the correct marketplace name (stellar-dev), aligning user-facing docs with .claude-plugin/marketplace.json so installs don’t fail with “Marketplace not found”.

Changes:

• Update the README install command to /plugin install stellar-dev@stellar-dev.
• Update the site’s “Installing” card command to match the corrected marketplace name.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
site/src/data/installers.mjs	Fixes the Claude Code installer command shown on the landing page/llms.txt to use the correct marketplace name.
README.md	Fixes the documented Claude Code install command to reference the actual marketplace name.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[kaankacar]

Verified against Codex's loader source. Discovery is recursive up to depth 6, so the clone-into-subdir form you proposed works correctly; Codex will find each skills/<name>/SKILL.md under ~/.codex/skills/stellar-dev-skill/. Confirmed against an actual Codex session: ~/.codex/skills/ is still the active read path for existing installs.

The path is marked "deprecated" in Codex source in favor of ~/.agents/skills/, but both paths still work and the rest of the README uses ~/.codex/skills/, so leaving it consistent for this PR seems right. A separate follow-up could modernize all Codex paths if we want, in the future.