feat: implement agent-notify CLI with hooks and tmux passthrough

Add OSC 777 notification sender, Cursor/Claude hook adapters,
config/install commands, and README for Ghostty + tmux setup.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/notify/osc.go

@@ -0,0 +1,16 @@
+package notify
+
+import "strings"
+
+func BuildSequence(protocol, title, body string) string {
+	switch protocol {
+	case "osc9":
+		return "\033]9;" + escapeOSCField(body) + "\007"
+	default:
+		return "\033]777;notify;" + escapeOSCField(title) + ";" + escapeOSCField(body) + "\007"
+	}
+}
+
+func escapeOSCField(s string) string {
+	return strings.ReplaceAll(s, ";", "\\;")
+}