feat(api): multi-project folder manifest for daily brief

Add build_brief_multi_project_manifest() to deep_agent.py that fetches
all project folder manifests via execute_on_client and keeps the top 5
most-recently-modified files per project. Wire into run_home_brief in
brief_agent.py, injecting the <linked_folders> block into the system
prompt alongside FOLDER_TOOLS.

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

tests/test_manifest_injection.py

@@ -1,7 +1,13 @@
 from __future__ import annotations
 
+from unittest.mock import AsyncMock, patch
+
+import pytest
+
 from app.core.deep_agent import format_folder_manifest, MANIFEST_TOKEN_BUDGET
 
+pytestmark = pytest.mark.asyncio
+
 
 def test_format_folder_manifest_basic():
     manifest = {
@@ -33,3 +39,31 @@ def test_format_folder_manifest_truncates_past_budget():
 def test_format_folder_manifest_null_returns_empty():
     assert format_folder_manifest(None) == ""
     assert format_folder_manifest({"files": []}) == ""
+
+
+async def test_brief_multi_project_manifest_top_5_per_project():
+    fake_response = [
+        {
+            "projectId": "p1", "projectName": "Acme", "folderPath": "/a",
+            "lastScannedAt": "now",
+            "files": [
+                {"relPath": f"f{i}.md", "kind": "text", "summary": "s", "mtimeMs": i}
+                for i in range(10)
+            ],
+        },
+        {
+            "projectId": "p2", "projectName": "Beta", "folderPath": "/b",
+            "lastScannedAt": "now",
+            "files": [{"relPath": "x.md", "kind": "text", "summary": "s", "mtimeMs": 1}],
+        },
+    ]
+    with patch(
+        "app.core.deep_agent.execute_on_client",
+        new=AsyncMock(return_value={"projects": fake_response}),
+    ):
+        from app.core.deep_agent import build_brief_multi_project_manifest
+        out = await build_brief_multi_project_manifest()
+    # Project 1 has 10 files, only top 5 by mtimeMs should appear
+    assert out.count("[p1]") <= 5
+    # Project 2 has 1 file, must appear
+    assert "[p2]" in out or "Beta" in out