ci: add cross-platform release workflow with version injection

Build linux/darwin amd64 and arm64 artifacts on push, upload GitHub
Release assets on v* tags, and expose version via agent-notify version.

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

cmd/agent-notify/version.go

@@ -0,0 +1,16 @@
+package main
+
+import "fmt"
+
+var (
+	version   = "dev"
+	commit    = "none"
+	buildDate = "unknown"
+)
+
+func cmdVersion() error {
+	fmt.Printf("agent-notify %s\n", version)
+	fmt.Printf("commit: %s\n", commit)
+	fmt.Printf("built: %s\n", buildDate)
+	return nil
+}