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/briefing_scheduler.py

@@ -444,7 +444,7 @@ async def _run_profile_closeout(user_id: int, model: str) -> None:
     and append them to the briefing profile note.
     """
     from fabledassistant.services.user_profile import append_observations
-    from fabledassistant.services.briefing_pipeline import _llm_synthesise
+    from fabledassistant.services.llm import generate_completion
     from fabledassistant.models.conversation import Conversation, Message
 
     yesterday = (date.today() - timedelta(days=1))
@@ -477,7 +477,17 @@ async def _run_profile_closeout(user_id: int, model: str) -> None:
         "Example: '- User skipped news about finance', '- Prefers weather for home location first'. "
         "If nothing notable, output only: (nothing to note)"
     )
-    observations = await _llm_synthesise(system, transcript, model)
+    try:
+        observations = (await generate_completion(
+            [
+                {"role": "system", "content": system},
+                {"role": "user", "content": transcript},
+            ],
+            model,
+        )).strip()
+    except Exception:
+        logger.warning("Profile closeout synthesis failed for user %d", user_id, exc_info=True)
+        observations = ""
     if observations and "(nothing to note)" not in observations.lower():
         await append_observations(user_id, observations)