docs(sdk): schéma project-plugins.json v1 + assignation skills/tools agents — #283 (project-plugin-assignments: workflow install→enable→assign→relaunch, schéma complet plugins/agents/skills/tools camelCase, ordre d'injection natif puis plugin, tombstones inertes uninstall/reinstall, validation UI/API idempotente vs édition manuelle, reload plugin ≠ relance agent; liens croisés manifest/codex-skills/README; QA verte: npm run build, liens docs vérifiés)

docs/manifest.md

@@ -177,6 +177,10 @@ Skill contribution rules:
 - Installing a plugin globally is not enough: a project must enable the plugin,
   then assign the skill to the target agent.
 
+See [Project Plugin Assignments](project-plugin-assignments.md) for the
+`.ideai/project-plugins.json` schema, validation rules, assignment order, and
+agent relaunch requirements.
+
 `contributes.layouts` is the only surface for plugin layouts. The same layout
 contribution can be mounted inside an IdeA layout cell or opened in a detached OS
 window. Do not add a separate manifest contribution type for windows.