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/brief_agent.py

@@ -21,6 +21,7 @@ from app.core.deep_agent import (
     _relational_memory_injection,
     _run_single_agent_stream,
     _trace_id_from_context,
+    build_brief_multi_project_manifest,
 )
 from app.core.langfuse_client import compile_prompt, get_prompt_or_fallback
 
@@ -159,6 +160,8 @@ async def run_home_brief(
     Yields (event_type, data) tuples identical to _run_single_agent_stream.
     Do NOT post-process output through _normalize_tagged_list_lines.
     """
+    from app.agents.folder_agent import FOLDER_TOOLS
+
     trace_id = _trace_id_from_context(context)
     today = date.today().isoformat()
     language = _resolve_language(context)
@@ -171,7 +174,10 @@ async def run_home_brief(
     if today not in system_prompt:
         system_prompt += f"\nToday is {today}."
 
-    tools = _build_read_tools(user_id, trace_id)
+    brief_manifest = await build_brief_multi_project_manifest()
+    system_prompt = system_prompt + ("\n\n" + brief_manifest if brief_manifest else "")
+
+    tools = [*_build_read_tools(user_id, trace_id), *FOLDER_TOOLS]
     async for event in _run_single_agent_stream(
         user_id=user_id,
         system_prompt=system_prompt,