Feat v0.8.1 workspace module

Managed disk directories for extensions needing real filesystem access
(git, compilers, media tools). Five builtins: create, path, list,
delete, usage. Extension-scoped to {WORKSPACE_ROOT}/{packageID}/{name}/.

- New sandbox/workspace_module.go with path traversal + symlink protection
- Permission: workspace.manage; config: WORKSPACE_ROOT, WORKSPACE_QUOTA_MB
- Runner wiring with graceful degradation (module absent if root unwritable)
- 16 new tests, all passing with -race

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

server/models/models_extension_perm.go

@@ -39,6 +39,7 @@ const (
 	ExtPermBatchExec            = "batch.exec"
 	ExtPermFilesRead            = "files.read"
 	ExtPermFilesWrite           = "files.write"
+	ExtPermWorkspaceManage      = "workspace.manage"
 )
 
 // ValidExtensionPermissions is the set of recognized permission keys.
@@ -56,6 +57,7 @@ var ValidExtensionPermissions = map[string]bool{
 	ExtPermBatchExec:            true,
 	ExtPermFilesRead:            true,
 	ExtPermFilesWrite:           true,
+	ExtPermWorkspaceManage:      true,
 }
 
 // ── Extension Permission Model ───────────────