feat(outlinekit): add parentDocumentId filter to listDocuments

documents.list supports it directly (per the vendored spec) — backs
fetching a document's direct children for the reader's sub-documents
section, without a second round-trip through the whole collection's
flat list.

OutlineKit/Sources/OutlineKit/Core/OutlineAPIClient.swift

@@ -8,7 +8,7 @@ public protocol OutlineAPIClient: Sendable {
     func authInfo() async throws -> OutlineAuthInfo
 
     func documentInfo(id: String) async throws -> OutlineDocument
-    func listDocuments(collectionId: String?, offset: Int, limit: Int) async throws -> [OutlineDocument]
+    func listDocuments(collectionId: String?, parentDocumentId: String?, offset: Int, limit: Int) async throws -> [OutlineDocument]
     /// Full-text search with snippets/ranking. Backed by `documents.search`.
     func searchDocuments(_ request: DocumentSearchRequest) async throws -> [OutlineDocumentSearchResult]
     /// Title-only search — faster, no snippets. Backed by `documents.search_titles`.

OutlineKit/Sources/OutlineKit/LiveOutlineAPIClient.swift

@@ -34,10 +34,20 @@ public actor LiveOutlineAPIClient: OutlineAPIClient {
         try await post("documents.info", body: DocumentInfoParams(id: id))
     }
 
-    public func listDocuments(collectionId: String?, offset: Int, limit: Int) async throws -> [OutlineDocument] {
+    public func listDocuments(
+        collectionId: String?,
+        parentDocumentId: String?,
+        offset: Int,
+        limit: Int
+    ) async throws -> [OutlineDocument] {
         try await post(
             "documents.list",
-            body: DocumentListParams(collectionId: collectionId, offset: offset, limit: limit)
+            body: DocumentListParams(
+                collectionId: collectionId,
+                parentDocumentId: parentDocumentId,
+                offset: offset,
+                limit: limit
+            )
         )
     }
 
@@ -188,6 +198,7 @@ private struct DocumentInfoParams: Encodable {
 
 private struct DocumentListParams: Encodable {
     let collectionId: String?
+    let parentDocumentId: String?
     let offset: Int
     let limit: Int
 }

OutlineKit/Tests/OutlineKitTests/LiveOutlineAPIClientTests.swift

@@ -257,6 +257,29 @@ final class LiveOutlineAPIClientTests: XCTestCase {
         XCTAssertEqual(decodedBody.description, "Updated overview text.")
     }
 
+    func testListDocumentsSendsParentDocumentIdFilter() async throws {
+        let httpClient = MockHTTPClient()
+        httpClient.responseData = """
+        { "data": [] }
+        """.data(using: .utf8)!
+
+        let client = LiveOutlineAPIClient(
+            configuration: OutlineConfiguration(baseURL: URL(string: "https://outline.example.com")!),
+            tokenStore: StaticTokenStore(),
+            httpClient: httpClient
+        )
+
+        _ = try await client.listDocuments(collectionId: nil, parentDocumentId: "doc-1", offset: 0, limit: 100)
+
+        struct SentBody: Decodable {
+            let parentDocumentId: String?
+        }
+
+        let sentBody = try XCTUnwrap(httpClient.lastRequest?.httpBody)
+        let decodedBody = try JSONDecoder().decode(SentBody.self, from: sentBody)
+        XCTAssertEqual(decodedBody.parentDocumentId, "doc-1")
+    }
+
     func testDocumentInfoDecodesEnvelopeAndSetsAuthHeader() async throws {
         let httpClient = MockHTTPClient()
         httpClient.responseData = """