feat(04-01): integrate notifications and focus mode into daemon

- Notifier + FocusTimer fields in Daemon, initialized in NewDaemon
- processHookEvent notifies on Working -> Needs Input only (D-01)
- Focus mode suppresses notifications when active (D-05)
- FocusArgs in protocol, "focus" handler in daemon
- CLI "vmux focus <minutes>" command
- Tests for all notification transitions and focus handler

protocol.go

@@ -13,9 +13,10 @@ type Request struct {
 
 // Response is the JSON message sent back by the daemon.
 type Response struct {
-	OK       bool          `json:"ok"`
-	Error    string        `json:"error,omitempty"`
-	Sessions []SessionInfo `json:"sessions,omitempty"`
+	OK             bool          `json:"ok"`
+	Error          string        `json:"error,omitempty"`
+	Sessions       []SessionInfo `json:"sessions,omitempty"`
+	FocusRemaining float64       `json:"focus_remaining,omitempty"`
 }
 
 // SessionInfo is the wire format for a session in IPC responses.
@@ -42,3 +43,8 @@ type LabelArgs struct {
 	SessionID string `json:"session_id"`
 	Label     string `json:"label"`
 }
+
+// FocusArgs carries the duration for the focus command.
+type FocusArgs struct {
+	Minutes int `json:"minutes"`
+}