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.go

@@ -15,6 +15,7 @@ type Config struct {
 	Providers ProvidersConfig `json:"providers"`
 	Gateway   GatewayConfig   `json:"gateway"`
 	Tools     ToolsConfig     `json:"tools"`
+	Heartbeat HeartbeatConfig `json:"heartbeat"`
 	mu        sync.RWMutex
 }
 
@@ -96,6 +97,10 @@ type SlackConfig struct {
 	AllowFrom []string `json:"allow_from" env:"PICOCLAW_CHANNELS_SLACK_ALLOW_FROM"`
 }
 
+type HeartbeatConfig struct {
+	Enabled bool `json:"enabled" env:"PICOCLAW_HEARTBEAT_ENABLED"`
+}
+
 type ProvidersConfig struct {
 	Anthropic  ProviderConfig `json:"anthropic"`
 	OpenAI     ProviderConfig `json:"openai"`
@@ -211,6 +216,9 @@ func DefaultConfig() *Config {
 				},
 			},
 		},
+		Heartbeat: HeartbeatConfig{
+			Enabled: true,
+		},
 	}
 }