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>

cmd/picoclaw/main.go

@@ -660,7 +660,7 @@ func gatewayCmd() {
 		cfg.WorkspacePath(),
 		nil,
 		30*60,
-		true,
+		cfg.Heartbeat.Enabled,
 	)
 
 	channelManager, err := channels.NewManager(cfg, msgBus)