Refactor system schemas to kernel, update imports

Moved system-related JSON schemas to a new 'kernel' directory and updated all relevant TypeScript imports from '@objectstack/spec/system' to '@objectstack/spec/kernel'. Removed system/plugin and manifest documentation from docs. Added new kernel schemas (BillOfMaterials, ComposerRequest, ComposerResponse, ConflictReport, DependencyRequirement) and updated package exports to include the kernel entry point.

Author: hotlong

content/docs/references/system/manifest/Manifest.mdx

@@ -8,9 +8,7 @@
     "identity",
     "job",
     "license",
-    "manifest",
     "organization",
-    "plugin",
     "policy",
     "role",
     "tenant",