feat: US-021 - Heartbeat calls ExecuteHeartbeatWithTools

Verified US-021 acceptance criteria:
- checkHeartbeat() calls hs.executeHeartbeatWithTools(prompt) ✓
- ProcessHeartbeat function does not exist (already deleted) ✓
- Typecheck passes (go build ./... succeeds) ✓

Note: US-020 and US-021 were already implemented in previous commits.
The checkHeartbeat function prioritizes the new tool-supporting handler
(hs.onHeartbeatWithTools) over the legacy handler (hs.onHeartbeat).

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

.ralph/prd.json

@@ -313,8 +313,8 @@
         "go build ./... succeeds"
       ],
       "priority": 21,
-      "passes": false,
-      "notes": ""
+      "passes": true,
+      "notes": "Already implemented"
     }
   ]
 }