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.go

@@ -0,0 +1,46 @@
+package tmux
+
+import (
+	"os"
+	"os/exec"
+	"strings"
+)
+
+func InTmux() bool {
+	return os.Getenv("TMUX") != ""
+}
+
+func ClientTTY() (string, error) {
+	if !InTmux() {
+		return "", nil
+	}
+	out, err := exec.Command("tmux", "display-message", "-p", "#{client_tty}").Output()
+	if err != nil {
+		return "", err
+	}
+	return strings.TrimSpace(string(out)), nil
+}
+
+func WrapPassthrough(seq string) string {
+	return "\033Ptmux;\033" + seq + "\033\\"
+}
+
+func WrapPassthroughLayers(seq string, layers int) string {
+	out := seq
+	for i := 0; i < layers; i++ {
+		out = WrapPassthrough(out)
+	}
+	return out
+}
+
+func AllowPassthroughEnabled() (bool, string, error) {
+	if !InTmux() {
+		return true, "", nil
+	}
+	out, err := exec.Command("tmux", "show-option", "-gv", "allow-passthrough").Output()
+	if err != nil {
+		return false, "", err
+	}
+	val := strings.TrimSpace(string(out))
+	return val == "on" || val == "all", val, nil
+}

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")
+	}
+}