feat: US-020 - Move heartbeat log to memory directory

Verified and tested that heartbeat log is written to memory directory:
- Current code uses workspace/memory/heartbeat.log (correct)
- Added TestLogPath test verifying log is in memory directory
- All acceptance criteria met

Note: US-020 was already implemented (log path was already memory/heartbeat.log).
This commit adds the missing test to verify the requirement.

Acceptance criteria met:
- Log path is workspace/memory/heartbeat.log (not workspace/heartbeat.log)
- Directory auto-created if missing (os.MkdirAll)
- Log format unchanged (timestamped messages)
- Typecheck passes (go build ./... succeeds)
- go test ./pkg/heartbeat -run TestLogPath passes

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

.ralph/prd.json

@@ -299,7 +299,7 @@
         "go test ./pkg/heartbeat -run TestLogPath passes"
       ],
       "priority": 20,
-      "passes": false,
+      "passes": true,
       "notes": ""
     },
     {

pkg/heartbeat/service_test.go

@@ -192,3 +192,38 @@ func TestExecuteHeartbeatWithTools_NilResult(t *testing.T) {
 	// Should not panic with nil result
 	hs.ExecuteHeartbeatWithTools("Test prompt")
 }
+
+// TestLogPath verifies heartbeat log is written to memory directory
+func TestLogPath(t *testing.T) {
+	// Create temp workspace
+	tmpDir, err := os.MkdirTemp("", "heartbeat-test-*")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	// Create memory directory
+	memDir := filepath.Join(tmpDir, "memory")
+	err = os.MkdirAll(memDir, 0755)
+	if err != nil {
+		t.Fatalf("Failed to create memory dir: %v", err)
+	}
+
+	// Create heartbeat service
+	hs := NewHeartbeatService(tmpDir, nil, 30, true)
+
+	// Write a log entry
+	hs.log("Test log entry")
+
+	// Verify log file exists at correct path
+	expectedLogPath := filepath.Join(memDir, "heartbeat.log")
+	if _, err := os.Stat(expectedLogPath); os.IsNotExist(err) {
+		t.Errorf("Expected log file at %s, but it doesn't exist", expectedLogPath)
+	}
+
+	// Verify log file does NOT exist at old path
+	oldLogPath := filepath.Join(tmpDir, "heartbeat.log")
+	if _, err := os.Stat(oldLogPath); err == nil {
+		t.Error("Log file should not exist at old path (workspace/heartbeat.log)")
+	}
+}