feat(cli): files kind in manifest + reconcile (§11.6 files C4)

- manifest: add CollectionKind::Files (+ "files" in as_str()); the
  string-or-table `collections` form now accepts { name, kind = "files" }.
- apply_service: add "files" to COLLECTION_KINDS. The rest of the
  reconcile (CollectionSpec, diff_collections, validate_bundle,
  state_token, the app-owner rejection) is already kind-generic.
- `pic collections ls` shows the files kind with no code change.

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

crates/manager-core/src/apply_service.rs

@@ -110,7 +110,7 @@ fn default_collection_kind() -> String {
 }
 
 /// The shared-collection kinds the apply engine accepts.
-const COLLECTION_KINDS: &[&str] = &["kv", "docs"];
+const COLLECTION_KINDS: &[&str] = &["kv", "docs", "files"];
 
 #[derive(Debug, Clone, Deserialize)]
 pub struct BundleScript {