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/send.go

@@ -0,0 +1,71 @@
+package notify
+
+import (
+	"fmt"
+	"io"
+	"os"
+
+	"github.com/longbin/agent-notify/internal/tmux"
+)
+
+type SendOptions struct {
+	Protocol  string
+	Title     string
+	Body      string
+	Writer    io.Writer
+	InTmux    bool
+	Layers    int
+	ClientTTY string
+}
+
+func Send(opts SendOptions) error {
+	seq := BuildSequence(opts.Protocol, opts.Title, opts.Body)
+	w := opts.Writer
+	if w == nil {
+		w = os.Stdout
+	}
+
+	if opts.InTmux && opts.ClientTTY != "" {
+		f, err := os.OpenFile(opts.ClientTTY, os.O_WRONLY, 0)
+		if err == nil {
+			defer f.Close()
+			_, err = io.WriteString(f, seq)
+			return err
+		}
+	}
+
+	out := seq
+	if opts.InTmux {
+		layers := opts.Layers
+		if layers <= 0 {
+			layers = 1
+		}
+		out = tmux.WrapPassthroughLayers(seq, layers)
+	}
+	_, err := io.WriteString(w, out)
+	return err
+}
+
+func SendAuto(protocol, title, body string) error {
+	inTmux := tmux.InTmux()
+	clientTTY, _ := tmux.ClientTTY()
+	layers := 0
+	if inTmux {
+		layers = 1
+	}
+	return Send(SendOptions{
+		Protocol:  protocol,
+		Title:     title,
+		Body:      body,
+		InTmux:    inTmux,
+		Layers:    layers,
+		ClientTTY: clientTTY,
+	})
+}
+
+func TestNotification(title, body string) error {
+	if err := SendAuto("osc777", title, body); err != nil {
+		return fmt.Errorf("send test notification: %w", err)
+	}
+	return nil
+}