feat(fable-mcp): add reset_today_briefing tool

Wipes today's briefing messages (keeps the conversation row) and
optionally re-fires the compilation slot to regenerate. Pairs with
the new POST /api/briefing/reset-today backend route.

Also drops the stale briefing_mode reference from trigger_briefing's
docstring now that legacy mode is gone.

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

fable-mcp/fable_mcp/tools/briefing.py

@@ -67,6 +67,26 @@ async def trigger_briefing(
     return await client.post("/api/briefing/trigger", json={"slot": slot})
 
 
+async def reset_today_briefing(
+    client: FableClient,
+    *,
+    run_compilation: bool = True,
+) -> dict[str, Any]:
+    """Delete all messages in today's briefing and optionally regenerate.
+
+    Wipes the messages in today's briefing conversation (keeping the
+    conversation row), then, if ``run_compilation`` is True, fires the
+    compilation slot so a fresh briefing lands in its place.
+    """
+    reset = await client.post("/api/briefing/reset-today")
+    if not run_compilation:
+        return {"reset": reset, "triggered": None}
+    triggered = await client.post(
+        "/api/briefing/trigger", json={"slot": "compilation"}
+    )
+    return {"reset": reset, "triggered": triggered}
+
+
 # ── Generic conversations ───────────────────────────────────────────────────
 
 async def get_conversation(