feat(vars): VarsService + vars:: SDK + config capabilities

Scripts can now read group-inherited config via vars::get(key) / vars::all().

- shared: VarsService trait (read-only get/all) + NoopVarsService; added as
  the 14th field on the Services bundle.
- manager-core: VarsServiceImpl resolves the calling app's config via
  config_resolver (derives app_id from cx.app_id; AppVarsRead-gated for
  authed principals, script-as-gate for anon).
- executor-core: vars:: Rhai bridge (get/all), registered in the SDK.
- authz: AppVarsRead/Write, GroupVarsRead/Write, GroupSecretsRead/Write
  capabilities (group caps resolve via the Phase-2 ancestor walk; the
  GroupSecretsRead human-read gate is group_admin-only, distinct from an
  app's runtime injection).
- wired VarsServiceImpl into the picloud binary; updated the executor-core
  test Services::new call sites.

386 manager-core lib tests green; clippy clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/manager-core/src/lib.rs

@@ -85,6 +85,7 @@ pub mod trigger_repo;
 pub mod triggers_api;
 pub mod users_admin_api;
 pub mod users_service;
+pub mod vars_service;
 
 pub use abandoned_repo::{
     AbandonedRepo, AbandonedRepoError, NewAbandonedExecution, PostgresAbandonedRepo,
@@ -220,3 +221,4 @@ pub use trigger_repo::{
 pub use triggers_api::{triggers_router, TriggersApiError, TriggersState};
 pub use users_admin_api::{app_users_router, AppUsersApiError, AppUsersState};
 pub use users_service::{UsersServiceConfig, UsersServiceImpl};
+pub use vars_service::VarsServiceImpl;