Prioritize AI marketplace installation docs

Constraint: The marketplace README is the primary install surface for AI agents and must stay synchronized with published plugins.

Rejected: Keep README as a secondary overview | stale install guidance makes plugin discovery unreliable.

Confidence: high

Scope-risk: narrow

Directive: Update README whenever public skills/plugins or plugin capabilities change.

Tested: python scripts/validate_marketplace.py; README plugin coverage check; temp CODEX_HOME marketplace add/list/plugin add docx.

Not-tested: Remote Git marketplace install; local checkout install exercises the same marketplace metadata.

plugins/codex/README.md

@@ -0,0 +1,24 @@
+# Codex Marketplace Sparse Entry
+
+This directory is the sparse checkout entry for the EAPIL Codex Plugin Marketplace. AI agents should normally install the marketplace from the repository root URL:
+
+```bash
+codex plugin marketplace add https://git.playones.com/arechen/EapilSkillMarket.git --ref main
+codex plugin list --marketplace eapil-skill-market
+codex plugin add <plugin-name>@eapil-skill-market
+```
+
+Use sparse checkout only when the environment needs a smaller checkout:
+
+```bash
+codex plugin marketplace add https://git.playones.com/arechen/EapilSkillMarket.git --ref main --sparse .agents --sparse plugins/codex
+```
+
+Local debug from the repository root:
+
+```bash
+codex plugin marketplace add .
+codex plugin add docx@eapil-skill-market
+```
+
+Each `plugins/<plugin-name>` directory is an installable Codex plugin. Plugins can include plain skills plus official Codex plugin components such as `hooks`, `.mcp.json`, and `.app.json`. See the root `README.md` for the full plugin table and maintenance rules.