Add task brief research agent: Stage 1 deep-research + canvas draft emission

- run_task_brief_research() runner with brief-specific tool set and max_steps=12
- New agents: client_agent (list_clients, get_client) and relations_agent (query_relations)
- search_associative tool wrapping MemoryMiddleware semantic search
- BRIEF_RESEARCH_TOOLS constant: read-only task/project/note/timeline + memory + client/relations
- canvas block extraction in output_formatter (splits visible text from <canvas> draft)
- device_ws.py: task_brief_research request type; emits canvas_draft mutation on stream_end
- Stage 2 briefMode: briefing_context injected into floating system prompt when present
- briefingContext kwarg wired through compile_prompt call chain

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

app/core/output_formatter.py

@@ -2,11 +2,35 @@
 
 from __future__ import annotations
 
+import re
 from collections.abc import AsyncGenerator
 from typing import Any
 
 from app.schemas import WsFloatingDomain, WsStreamEnd, WsStreamStart, WsStreamText
 
+# Matches <canvas kind="...">...</canvas> blocks (single-line or multiline).
+_CANVAS_BLOCK_RE = re.compile(
+    r'<canvas\s+kind=["\']([^"\']+)["\']>(.*?)</canvas>',
+    re.DOTALL | re.IGNORECASE,
+)
+
+
+def extract_canvas_block(text: str) -> tuple[str, str | None, str | None]:
+    """Strip the first <canvas kind="...">...</canvas> block from *text*.
+
+    Returns ``(visible_text, canvas_content, canvas_kind)``.
+    ``canvas_content`` and ``canvas_kind`` are ``None`` when no block is found.
+    """
+    match = _CANVAS_BLOCK_RE.search(text)
+    if not match:
+        return text, None, None
+
+    canvas_kind = match.group(1).strip()
+    canvas_content = match.group(2).strip()
+    visible = text[: match.start()] + text[match.end() :]
+    visible = visible.strip()
+    return visible, canvas_content, canvas_kind
+
 WsFrame = WsStreamStart | WsStreamText | WsStreamEnd | WsFloatingDomain