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.

crates/core-api/src/config_property.rs

@@ -27,6 +27,9 @@ pub enum PropertyType {
     SecurityGroup,
     /// Dropdown of the interface languages the instance supports.
     Locale,
+    /// Dropdown of the LLM models configured on the instance (by model name,
+    /// the resolution key). Nullable: empty means "auto-select".
+    LlmModel,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]

crates/core-api/src/provider.rs

@@ -44,7 +44,6 @@ pub struct LlmModelRecord {
     pub model_id:          String,
     pub name:              String,
     pub strength:          Option<LlmStrength>,
-    pub scope:             Vec<String>,
     pub is_default:        bool,
     pub priority:          i32,
     pub extra_params:      Option<serde_json::Value>,