feat(commands): /background, /btw slash commands + undo button + reasoning chip

Rebased onto master after #931 (aux title routing) to resolve streaming.py conflict.
All changes from both PRs are cleanly integrated.

2088 tests passing (2065 master + 23 from #931).

Co-authored-by: bergeouss <bergeouss@gmail.com>

api/background.py

@@ -0,0 +1,87 @@
+"""Background and ephemeral task tracking for /background and /btw commands."""
+from __future__ import annotations
+
+import logging
+import threading
+import time
+from typing import Any
+
+logger = logging.getLogger(__name__)
+
+_lock = threading.Lock()
+
+# parent_session_id -> list of task dicts
+_BACKGROUND_TASKS: dict[str, list[dict[str, Any]]] = {}
+
+# btw ephemeral session tracking: parent_sid -> {ephemeral_sid, stream_id, question}
+_BTW_TRACKING: dict[str, dict[str, Any]] = {}
+
+
+def track_background(parent_sid: str, bg_sid: str, stream_id: str,
+                     task_id: str, prompt: str) -> None:
+    with _lock:
+        _BACKGROUND_TASKS.setdefault(parent_sid, []).append({
+            "task_id": task_id,
+            "bg_session_id": bg_sid,
+            "stream_id": stream_id,
+            "prompt": prompt,
+            "status": "running",
+            "started_at": time.time(),
+            "answer": None,
+            "completed_at": None,
+        })
+
+
+def track_btw(parent_sid: str, ephemeral_sid: str, stream_id: str,
+              question: str) -> None:
+    with _lock:
+        _BTW_TRACKING[parent_sid] = {
+            "ephemeral_session_id": ephemeral_sid,
+            "stream_id": stream_id,
+            "question": question,
+        }
+
+
+def complete_background(parent_sid: str, task_id: str, answer: str) -> None:
+    with _lock:
+        for t in _BACKGROUND_TASKS.get(parent_sid, []):
+            if t["task_id"] == task_id and t["status"] == "running":
+                t["status"] = "done"
+                t["answer"] = answer
+                t["completed_at"] = time.time()
+                break
+
+
+def get_results(parent_sid: str) -> list[dict[str, Any]]:
+    """Return completed background task results and remove only the done ones
+    from tracking.  Tasks still in ``status="running"`` MUST stay in the list
+    so that ``complete_background()`` can still find them when the worker
+    thread finishes — otherwise the first poll during a long-running task
+    silently drops it and the result is lost forever.
+    """
+    with _lock:
+        tasks = _BACKGROUND_TASKS.get(parent_sid, [])
+        done = [t for t in tasks if t["status"] == "done"]
+        still_running = [t for t in tasks if t["status"] != "done"]
+        if still_running:
+            _BACKGROUND_TASKS[parent_sid] = still_running
+        else:
+            _BACKGROUND_TASKS.pop(parent_sid, None)
+        return [{
+            "task_id": t["task_id"],
+            "prompt": t["prompt"],
+            "answer": t["answer"],
+            "completed_at": t["completed_at"],
+        } for t in done]
+
+
+def get_background_tasks(parent_sid: str) -> list[dict[str, Any]]:
+    """Return all background tasks (running and done) for a parent session."""
+    with _lock:
+        return list(_BACKGROUND_TASKS.get(parent_sid, []))
+
+
+def cleanup_btw(parent_sid: str) -> dict[str, Any] | None:
+    """Remove and return btw tracking for a parent session."""
+    with _lock:
+        return _BTW_TRACKING.pop(parent_sid, None)