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>

Makefile

@@ -1,11 +1,28 @@
-.PHONY: build test install
+.PHONY: build test install cross
 BINARY := agent-notify
+VERSION ?= dev
+COMMIT ?= $(shell git rev-parse --short HEAD 2>/dev/null || echo none)
+BUILD_DATE ?= $(shell date -u +%Y-%m-%dT%H:%M:%SZ)
+LDFLAGS := -s -w -X main.version=$(VERSION) -X main.commit=$(COMMIT) -X main.buildDate=$(BUILD_DATE)
 
 build:
-	go build -o bin/$(BINARY) ./cmd/agent-notify
+	go build -ldflags "$(LDFLAGS)" -o bin/$(BINARY) ./cmd/agent-notify
 
 test:
 	go test ./...
 
 install: build
 	install -m 755 bin/$(BINARY) $(HOME)/.local/bin/$(BINARY)
+
+cross:
+	@mkdir -p dist
+	@for target in \
+		"linux amd64" \
+		"linux arm64" \
+		"darwin amd64" \
+		"darwin arm64"; do \
+		set -- $$target; \
+		out=dist/$(BINARY)-$(VERSION)_$$1_$$2; \
+		echo "building $$out"; \
+		GOOS=$$1 GOARCH=$$2 CGO_ENABLED=0 go build -trimpath -ldflags "$(LDFLAGS)" -o "$$out" ./cmd/agent-notify; \
+	done