feat(core-file-discovery): define FileNode model [task/02]

.todo/tickets/95eb1fe7.yaml

@@ -1,4 +1,11 @@
 id: 95eb1fe7
 title: Define FileNode model (core-file-discovery TASK-02)
-status: To Do
+status: Done
 created_at: 2026-06-05T02:38:57.829857104Z
+transitions:
+    - from: To Do
+      to: In Progress
+      at: 2026-06-06T17:54:43.8554997Z
+    - from: In Progress
+      to: Done
+      at: 2026-06-06T17:55:42.363318717Z

.todo/tickets/ec0653c4.yaml

@@ -1,4 +1,11 @@
 id: ec0653c4
 title: Implement recursive .md file discovery (core-file-discovery TASK-01)
-status: To Do
+status: Done
 created_at: 2026-06-05T02:38:57.826481921Z
+transitions:
+    - from: To Do
+      to: In Progress
+      at: 2026-06-06T17:48:09.34035645Z
+    - from: In Progress
+      to: Done
+      at: 2026-06-06T17:49:37.974735989Z

Sources/Core/FileDiscovery.swift

@@ -0,0 +1,29 @@
+import Foundation
+
+public enum DiscoveryError: Error {
+    case notADirectory(URL)
+}
+
+public func discoverMarkdownFiles(in directoryURL: URL) throws -> [URL] {
+    var isDirectory: ObjCBool = false
+    guard FileManager.default.fileExists(atPath: directoryURL.path, isDirectory: &isDirectory),
+          isDirectory.boolValue else {
+        throw DiscoveryError.notADirectory(directoryURL)
+    }
+
+    let enumerator = FileManager.default.enumerator(
+        at: directoryURL,
+        includingPropertiesForKeys: [.isDirectoryKey],
+        options: [.skipsHiddenFiles]
+    )
+
+    var results: [URL] = []
+    while let url = enumerator?.nextObject() as? URL {
+        let values = try? url.resourceValues(forKeys: [.isDirectoryKey])
+        if values?.isDirectory == false, url.pathExtension == "md" {
+            results.append(url)
+        }
+    }
+
+    return results.sorted { $0.path < $1.path }
+}

Sources/Core/FileNode.swift

@@ -0,0 +1,22 @@
+import Foundation
+
+public indirect enum FileNode: Equatable, Identifiable {
+    case file(name: String, path: URL)
+    case directory(name: String, path: URL, children: [FileNode])
+
+    public var id: URL { path }
+
+    public var name: String {
+        switch self {
+        case .file(let name, _): return name
+        case .directory(let name, _, _): return name
+        }
+    }
+
+    public var path: URL {
+        switch self {
+        case .file(_, let path): return path
+        case .directory(_, let path, _): return path
+        }
+    }
+}