Feat v0.8.0 files sandbox module

Bridge ObjectStore (PVC/S3) into the Starlark sandbox with extension-scoped
key namespacing. New files module with put/get/meta/list/delete/exists
builtins gated by files.read and files.write permissions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/models/models_extension_perm.go

@@ -37,6 +37,8 @@ const (
 	ExtPermTriggersRegister     = "triggers.register"
 	ExtPermRealtimePublish      = "realtime.publish"
 	ExtPermBatchExec            = "batch.exec"
+	ExtPermFilesRead            = "files.read"
+	ExtPermFilesWrite           = "files.write"
 )
 
 // ValidExtensionPermissions is the set of recognized permission keys.
@@ -52,6 +54,8 @@ var ValidExtensionPermissions = map[string]bool{
 	ExtPermTriggersRegister:     true,
 	ExtPermRealtimePublish:      true,
 	ExtPermBatchExec:            true,
+	ExtPermFilesRead:            true,
+	ExtPermFilesWrite:           true,
 }
 
 // ── Extension Permission Model ───────────────