fix: join root task sections with newlines

build_root_task assembled section headers and per-item bullets as
separate list elements, then joined them with a single space. Scans
with more than one target (or diff-scope constraints) rendered every
bullet on one run-on line, e.g. "URLs: - a - b" instead of a proper
list. Join with a newline to match the sibling child_initial_input
builder. Add a regression test covering multiple targets.

Closes #682

strix/core/inputs.py

@@ -73,7 +73,7 @@ def build_root_task(scan_config: dict[str, Any]) -> str:
             if deleted:
                 parts.append(f"- {label}: {deleted} deleted file(s) are context-only")
 
-    task = " ".join(parts)
+    task = "\n".join(parts)
     if user_instructions:
         task = f"{task}\n\nSpecial instructions: {user_instructions}"
     return task

tests/test_inputs.py

@@ -93,6 +93,20 @@ def test_build_root_task_web_application_with_instructions() -> None:
     assert "Special instructions: Focus on auth." in task
 
 
+def test_build_root_task_multiple_targets_are_newline_separated() -> None:
+    config = {
+        "targets": [
+            {"type": "web_application", "details": {"target_url": "https://a.example.com"}},
+            {"type": "web_application", "details": {"target_url": "https://b.example.com"}},
+        ],
+    }
+    task = build_root_task(config)
+
+    assert "\n- https://a.example.com" in task
+    assert "\n- https://b.example.com" in task
+    assert "https://a.example.com - https://b.example.com" not in task
+
+
 def test_build_root_task_diff_scope() -> None:
     config = {
         "targets": [],