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/tmux/tmux_test.go

@@ -0,0 +1,29 @@
+package tmux
+
+import "testing"
+
+func TestInTmux(t *testing.T) {
+	t.Setenv("TMUX", "/tmp/tmux-123,1,0")
+	if !InTmux() {
+		t.Fatal("expected InTmux true")
+	}
+}
+
+func TestWrapPassthroughSingle(t *testing.T) {
+	inner := "\033]777;notify;t;b\007"
+	got := WrapPassthrough(inner)
+	want := "\033Ptmux;\033" + inner + "\033\\"
+	if got != want {
+		t.Fatalf("got %q want %q", got, want)
+	}
+}
+
+func TestWrapPassthroughNested(t *testing.T) {
+	inner := "\033]777;notify;t;b\007"
+	got := WrapPassthroughLayers(inner, 2)
+	once := WrapPassthrough(inner)
+	twice := WrapPassthrough(once)
+	if got != twice {
+		t.Fatalf("nested wrap mismatch")
+	}
+}