refactor(briefing)!: remove legacy one-shot synthesis, agentic-only

Deletes ~760 lines of legacy briefing code: format_task, compute_task_hash,
upsert_task_snapshots, _gather_internal, _gather_weekly_review,
_llm_synthesise, and the unified prompt helpers. run_compilation and
run_slot_injection are now agentic-tool-use-loop only.

briefing_scheduler and user_profile migrated from the deleted helper to
services.llm.generate_completion (retry + keep_alive baked in).

routes/briefing.manual_trigger now persists agentic tool-call receipts
via _persist_agentic_messages (previously silently dropped them) and
adds POST /api/briefing/reset-today to wipe today's briefing messages.

BREAKING: briefing_mode setting no longer honored; no legacy fallback.

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

src/fabledassistant/routes/briefing.py

@@ -308,14 +308,52 @@ async def manual_trigger():
         logger.warning("Pre-trigger refresh failed for user %d", g.user.id, exc_info=True)
 
     from fabledassistant.services.briefing_pipeline import run_compilation
+    from fabledassistant.services.briefing_scheduler import _persist_agentic_messages
 
     model = await get_setting(g.user.id, "default_model", "")
     conv = await get_or_create_today_conversation(g.user.id, model)
     text, metadata = await run_compilation(g.user.id, slot, model)
-    msg = await post_message(conv.id, "assistant", text, metadata=metadata)
+    await _persist_agentic_messages(conv.id, slot, metadata)
+    final_meta = {k: v for k, v in metadata.items() if k != "agentic_messages"}
+    final_meta["briefing_slot"] = slot
+    msg = await post_message(conv.id, "assistant", text, metadata=final_meta)
     return jsonify({"conversation_id": conv.id, "message_id": msg.id, "slot": slot})
 
 
+@briefing_bp.route("/reset-today", methods=["POST"])
+@_REQUIRE
+async def reset_today_briefing():
+    """Delete all messages in today's briefing conversation.
+
+    The conversation row itself is kept so its id stays stable for any
+    open UI sessions. Intended for "wipe and start fresh" workflows
+    driven from the MCP when iterating on prompts. Pair with
+    ``POST /api/briefing/trigger`` to immediately regenerate.
+    """
+    from datetime import date as _date
+    from sqlalchemy import delete as _delete
+
+    today = _date.today()
+    async with async_session() as session:
+        result = await session.execute(
+            select(Conversation).where(
+                Conversation.user_id == g.user.id,
+                Conversation.conversation_type == "briefing",
+                Conversation.briefing_date == today,
+            )
+        )
+        conv = result.scalars().first()
+        if conv is None:
+            return jsonify({"deleted": 0, "conversation_id": None})
+        deleted_result = await session.execute(
+            _delete(Message).where(Message.conversation_id == conv.id)
+        )
+        await session.commit()
+        deleted = deleted_result.rowcount or 0
+
+    return jsonify({"deleted": deleted, "conversation_id": conv.id})
+
+
 # ── RSS Reactions ──────────────────────────────────────────────────────────────
 
 @briefing_bp.route("/rss-reactions", methods=["POST"])