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>

app/core/deep_agent.py

@@ -110,6 +110,35 @@ async def _fetch_project_manifest(project_id: str) -> dict | None:
         return None
 
 
+async def build_brief_multi_project_manifest() -> str:
+    """Build a compact multi-project manifest for the daily brief agent.
+
+    Calls execute_on_client('list_projects_with_folder_manifests') and keeps
+    the top 5 most-recently-modified files per project.
+    """
+    try:
+        result = await execute_on_client(
+            action="list_projects_with_folder_manifests",
+            data={},
+        )
+    except Exception:
+        return ""
+    projects = (result or {}).get("projects") or []
+    if not projects:
+        return ""
+    blocks: list[str] = ["<linked_folders>"]
+    for p in projects:
+        files = sorted(p.get("files", []), key=lambda f: f.get("mtimeMs", 0), reverse=True)[:5]
+        if not files:
+            continue
+        blocks.append(f"project: {p.get('projectName','?')} [{p.get('projectId','?')}]")
+        blocks.append(f"  path: {p.get('folderPath','?')}  (scanned {p.get('lastScannedAt','?')})")
+        for f in files:
+            blocks.append(f"  - /{f['relPath']}  [{f.get('kind','text')}]  {f.get('summary','')}")
+    blocks.append("</linked_folders>")
+    return "\n".join(blocks)
+
+
 def _datetime_context_injection(context: dict[str, Any]) -> str:
     """Build a comprehensive DATE CONTEXT block with pre-computed ms-epoch boundaries for common ranges."""
     fp = context.get("format_prefs")