feat(mac): add agent-controlled Canvas panel

apps/macos/Sources/Clawdis/CanvasFileWatcher.swift

@@ -0,0 +1,61 @@
+import Foundation
+import Darwin
+
+final class CanvasFileWatcher: @unchecked Sendable {
+    private let url: URL
+    private let queue: DispatchQueue
+    private var source: DispatchSourceFileSystemObject?
+    private var fd: Int32 = -1
+    private var pending = false
+    private let onChange: () -> Void
+
+    init(url: URL, onChange: @escaping () -> Void) {
+        self.url = url
+        self.queue = DispatchQueue(label: "com.steipete.clawdis.canvaswatcher")
+        self.onChange = onChange
+    }
+
+    deinit {
+        self.stop()
+    }
+
+    func start() {
+        guard self.source == nil else { return }
+        let path = (self.url as NSURL).fileSystemRepresentation
+        let fd = open(path, O_EVTONLY)
+        guard fd >= 0 else { return }
+        self.fd = fd
+
+        let source = DispatchSource.makeFileSystemObjectSource(
+            fileDescriptor: fd,
+            eventMask: [.write, .delete, .rename, .attrib, .extend, .link, .revoke],
+            queue: self.queue)
+
+        source.setEventHandler { [weak self] in
+            guard let self else { return }
+            if self.pending { return }
+            self.pending = true
+            self.queue.asyncAfter(deadline: .now() + 0.12) { [weak self] in
+                guard let self else { return }
+                self.pending = false
+                self.onChange()
+            }
+        }
+
+        source.setCancelHandler { [weak self] in
+            guard let self else { return }
+            if self.fd >= 0 {
+                close(self.fd)
+                self.fd = -1
+            }
+        }
+
+        self.source = source
+        source.resume()
+    }
+
+    func stop() {
+        self.source?.cancel()
+        self.source = nil
+    }
+}