Let Codex discover the marketplace from the repository root

Constraint: Codex Git sparse checkout preserves selected paths, so plugins/codex is not treated as the marketplace root by the CLI.\nRejected: Require users to rely on a single plugins/codex sparse path | The CLI leaves the manifest nested and rejects the marketplace root.\nConfidence: high\nScope-risk: narrow\nDirective: Keep the root .agents/plugins/marketplace.json as the primary Git entrypoint.\nTested: python json.tool on both marketplace manifests; validate_plugin.py.\nNot-tested: Remote Git marketplace add after this commit.

README.md

@@ -8,21 +8,35 @@ In **Add plugin marketplace**:
 
 - Source: `https://git.playones.com/arechen/EapilSkillMarket.git`
 - Git ref: `main`
-- Sparse path: `plugins/codex`
+- Sparse path: leave empty
 
 After the marketplace is added, install `eapil-skills` from the `EAPIL Skill Market` marketplace.
 
+If you want to use sparse checkout, enter both paths if the UI allows multiple lines:
+
+```text
+.agents
+plugins/codex
+```
+
 ## CLI test
 
 ```powershell
-codex plugin marketplace add https://git.playones.com/arechen/EapilSkillMarket.git --ref main --sparse plugins/codex
+codex plugin marketplace add https://git.playones.com/arechen/EapilSkillMarket.git --ref main
+codex plugin add eapil-skills@eapil-skill-market
+```
+
+Sparse CLI form:
+
+```powershell
+codex plugin marketplace add https://git.playones.com/arechen/EapilSkillMarket.git --ref main --sparse .agents --sparse plugins/codex
 codex plugin add eapil-skills@eapil-skill-market
 ```
 
 For local testing before pushing:
 
 ```powershell
-codex plugin marketplace add .\plugins\codex
+codex plugin marketplace add .
 codex plugin add eapil-skills@eapil-skill-market
 ```
 
@@ -31,6 +45,7 @@ Start a new Codex thread after installing so the new skills are loaded into cont
 ## Layout
 
 ```text
+.agents/plugins/marketplace.json
 plugins/codex/
   .agents/plugins/marketplace.json
   plugins/eapil-skills/