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/agents.css

@@ -145,16 +145,6 @@ agents-page {
   flex-shrink: 0;
 }
 
-.agent-scope-pill {
-  display: inline-block;
-  font-size: 0.7rem;
-  padding: 0.1rem 0.45rem;
-  border-radius: 999px;
-  background: var(--bs-secondary-bg);
-  color: var(--bs-secondary-color);
-  border: 1px solid var(--bs-border-color);
-}
-
 /* ── Detail view ─────────────────────────────────────────────────────────── */
 
 .agent-detail {