fix(desktop): add @mainactor thread safety to InputMonitor

Add proper thread safety to InputMonitor by using @mainactor isolation
instead of @unchecked Sendable with unprotected mutable dictionaries.

Changes:
- InputMonitor is now @mainactor isolated
- detectors array is explicitly nonisolated (immutable, safe)
- init is nonisolated for use as default parameter
- TerminalViewRepresentable.checkForInputPrompts() is @mainactor
- Timer callback wraps call in Task { @mainactor in ... }
- Add content hash caching to skip redundant input detection

This ensures compile-time guarantees that InputMonitor state is only
accessed from the main thread, preventing potential data races.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: grokify

apps/desktop/Sources/PlexusOneDesktop/Services/InputMonitor.swift

@@ -6,16 +6,22 @@ import UserNotifications
 
 /// Monitors terminal content for input prompts from AI assistants.
 /// Uses AssistantKit's pattern detectors to identify when the user needs to respond.
+///
+/// Thread Safety: This class is `@MainActor` isolated since it:
+/// - Updates UI-observable state (`activeAlerts`, `activeSuggestedActions`)
+/// - Posts NotificationCenter notifications
+/// - Interacts with AppKit (NSSound) and UserNotifications
+@MainActor
 @Observable
-final class InputMonitor: @unchecked Sendable {
+final class InputMonitor {
     /// Active alerts by session ID
     private(set) var activeAlerts: [UUID: DetectionResult] = [:]
 
     /// Suggested actions for active alerts
     private(set) var activeSuggestedActions: [UUID: [SuggestedAction]] = [:]
 
     /// Detectors to use for scanning terminal content
-    private let detectors: [any InputDetector]
+    private nonisolated let detectors: [any InputDetector]
 
     /// Whether to show macOS notifications
     var enableNotifications: Bool = false
@@ -26,7 +32,9 @@ final class InputMonitor: @unchecked Sendable {
     /// Minimum confidence threshold for alerts
     var confidenceThreshold: Double = 0.7
 
-    init(requestNotifications: Bool = true) {
+    /// Initialize InputMonitor.
+    /// - Parameter requestNotifications: Whether to request notification permissions (set to false in tests)
+    nonisolated init(requestNotifications: Bool = true) {
         self.detectors = [
             ClaudeDetector(),
             KiroDetector(),
@@ -35,7 +43,9 @@ final class InputMonitor: @unchecked Sendable {
 
         // Request notification permission if needed (skip in test environments)
         if requestNotifications {
-            requestNotificationPermission()
+            Task { @MainActor in
+                self.requestNotificationPermission()
+            }
         }
     }
 

apps/desktop/Sources/PlexusOneDesktop/Views/TerminalViewRepresentable.swift

@@ -178,6 +178,9 @@ struct AppTerminalViewRepresentable: NSViewRepresentable {
         var scrollMonitor: Any?
         var inputDetectionTimer: Timer?
 
+        /// Cache last content hash to skip redundant input detection
+        private var lastContentHash: Int = 0
+
         init(_ parent: AppTerminalViewRepresentable) {
             self.parent = parent
         }
@@ -192,12 +195,17 @@ struct AppTerminalViewRepresentable: NSViewRepresentable {
         /// Start periodic input detection and focus checking
         func startInputDetection() {
             // Poll for input prompts and focus state every 500ms
+            // Timer fires on main thread, checkForInputPrompts is @MainActor
             inputDetectionTimer = Timer.scheduledTimer(withTimeInterval: 0.5, repeats: true) { [weak self] _ in
-                self?.checkForInputPrompts()
-                self?.containerView?.updateFocusState()
+                guard let self = self else { return }
+                Task { @MainActor in
+                    self.checkForInputPrompts()
+                }
+                self.containerView?.updateFocusState()
             }
         }
 
+        @MainActor
         private func checkForInputPrompts() {
             guard let terminalView = terminalView,
                   let sessionId = terminalView.attachedSessionId() else {
@@ -208,6 +216,11 @@ struct AppTerminalViewRepresentable: NSViewRepresentable {
             let content = extractRecentContent(from: terminalView, lineCount: 15)
             guard !content.isEmpty else { return }
 
+            // Skip processing if content hasn't changed (performance optimization)
+            let contentHash = content.hashValue
+            guard contentHash != lastContentHash else { return }
+            lastContentHash = contentHash
+
             // Get cursor position
             guard let terminal = terminalView.terminal else { return }
             let cursor = terminal.getCursorLocation()
@@ -222,9 +235,7 @@ struct AppTerminalViewRepresentable: NSViewRepresentable {
 
             // Notify parent if input detected
             if let result = parent.inputMonitor.alert(for: sessionId) {
-                DispatchQueue.main.async { [weak self] in
-                    self?.parent.onInputDetected?(result)
-                }
+                parent.onInputDetected?(result)
             }
         }