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/services/user_profile.py

@@ -114,7 +114,7 @@ async def _consolidate_observations(user_id: int) -> str:
     learned_summary paragraph. Prunes raw entries older than 30 days afterwards.
     """
     from fabledassistant.config import Config
-    from fabledassistant.services.briefing_pipeline import _llm_synthesise
+    from fabledassistant.services.llm import generate_completion
     from fabledassistant.services.settings import get_setting
 
     async with async_session() as session:
@@ -145,7 +145,17 @@ async def _consolidate_observations(user_id: int) -> str:
     user_prompt += f"New observations:\n{obs_text}"
 
     model = await get_setting(user_id, "default_model", Config.OLLAMA_MODEL)
-    new_summary = await _llm_synthesise(system, user_prompt, model)
+    try:
+        new_summary = (await generate_completion(
+            [
+                {"role": "system", "content": system},
+                {"role": "user", "content": user_prompt},
+            ],
+            model,
+        )).strip()
+    except Exception:
+        logger.warning("Observation consolidation failed for user %d", user_id, exc_info=True)
+        new_summary = ""
 
     if new_summary:
         cutoff = (date.today() - timedelta(days=30)).isoformat()