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>

.ralph/prd.json

@@ -284,7 +284,7 @@
         "go test ./pkg/config -run TestDefaultConfig passes"
       ],
       "priority": 19,
-      "passes": false,
+      "passes": true,
       "notes": ""
     },
     {