feat: US-019 - Enable heartbeat by default in config

- Added HeartbeatConfig struct with Enabled field
- Added Heartbeat to Config struct
- Set default Heartbeat.Enabled = true in DefaultConfig()
- Updated main.go to use cfg.Heartbeat.Enabled instead of hardcoded true
- Added config tests verifying heartbeat is enabled by default

Acceptance criteria met:
- DefaultConfig() Heartbeat.Enabled changed to true
- Can override via PICOCLAW_HEARTBEAT_ENABLED=false env var
- Config documentation updated showing default enabled
- Typecheck passes (go build ./... succeeds)
- go test ./pkg/config -run TestDefaultConfig passes

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

pkg/config/config_test.go

@@ -0,0 +1,24 @@
+package config
+
+import (
+	"testing"
+)
+
+// TestDefaultConfig_HeartbeatEnabled verifies heartbeat is enabled by default
+func TestDefaultConfig_HeartbeatEnabled(t *testing.T) {
+	cfg := DefaultConfig()
+
+	if !cfg.Heartbeat.Enabled {
+		t.Error("Heartbeat should be enabled by default")
+	}
+}
+
+// TestDefaultConfig_HeartbeatCanBeDisabled verifies heartbeat can be disabled via config
+func TestDefaultConfig_HeartbeatCanBeDisabled(t *testing.T) {
+	cfg := &Config{}
+	cfg.Heartbeat.Enabled = false
+
+	if cfg.Heartbeat.Enabled {
+		t.Error("Heartbeat should be disabled when set to false")
+	}
+}