llm: drop model/agent scope matching; add instance-wide compaction model picker

Remove the scope system end-to-end (llm_models.scope column, agent meta
scope field, scope-based tier in model selection, UI checkboxes/pills):
it was only a soft ranking hint, had drifted (6 UI scopes vs 3 used by
agents, 'general' not even selectable) and duplicated what strength
already decides. Strength stays the single AUTO-selection axis.

Compaction: the summary model is now pickable from the Settings page
via a new PropertyType::LlmModel config property (registry key
compaction_model), instance-wide and live (no restart). Fallback chain:
explicit pick -> compaction.strength from config.yml -> priority order;
a deleted configured model degrades to AUTO. ContextCompactor reads the
key at compact time through GlobalConfigManager.

web/css/models-llm.css

@@ -356,14 +356,6 @@
   max-width: 680px;
 }
 
-/* ── Scope checkbox grid ── */
-
-.llm-scope-grid {
-  display: grid;
-  grid-template-columns: repeat(3, 1fr);
-  gap: 0.35rem 1rem;
-}
-
 /* ── Provider picker grid ── */
 
 .llm-provider-grid {