feat: merge heartbeat service improvements from feat-heartbeat branch

- Add ChannelSender interface for sending heartbeat results to users
- Add sendResponse() to automatically deliver results to last channel
- Add createDefaultHeartbeatTemplate() for first-run setup
- Support HEARTBEAT_OK silent response (legacy compatibility)
- Add structured logging with INFO/ERROR levels
- Move integration tests to separate file with build tag

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/heartbeat/service_test.go

@@ -213,7 +213,7 @@ func TestLogPath(t *testing.T) {
 	hs := NewHeartbeatService(tmpDir, nil, 30, true)
 
 	// Write a log entry
-	hs.log("Test log entry")
+	hs.log("INFO", "Test log entry")
 
 	// Verify log file exists at correct path
 	expectedLogPath := filepath.Join(memDir, "heartbeat.log")