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/picloud-cli/src/manifest.rs

@@ -269,6 +269,7 @@ pub struct ManifestGroup {
 pub enum CollectionKind {
     Kv,
     Docs,
+    Files,
 }
 
 impl CollectionKind {
@@ -277,6 +278,7 @@ impl CollectionKind {
         match self {
             Self::Kv => "kv",
             Self::Docs => "docs",
+            Self::Files => "files",
         }
     }
 }
@@ -726,7 +728,8 @@ mod tests {
         // `{ name, kind }` table sets an explicit kind. Both coexist.
         let m = Manifest::parse(
             "[group]\nslug = \"acme\"\nname = \"ACME\"\n\n\
-             collections = [\"catalog\", { name = \"articles\", kind = \"docs\" }]\n",
+             collections = [\"catalog\", { name = \"articles\", kind = \"docs\" }, \
+             { name = \"assets\", kind = \"files\" }]\n",
         )
         .expect("string-or-table collections must parse");
         assert_eq!(
@@ -734,6 +737,7 @@ mod tests {
             vec![
                 ("catalog".to_string(), "kv".to_string()),
                 ("articles".to_string(), "docs".to_string()),
+                ("assets".to_string(), "files".to_string()),
             ]
         );
         // An app manifest carrying `collections` is rejected (no such field +