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.

src/frontend/api/llm.rs

@@ -162,7 +162,6 @@ pub struct ModelPayload {
     pub model_id:          String,
     pub name:              String,
     pub strength:          Option<String>,
-    pub scope:             Option<Vec<String>>,
     pub is_default:        Option<bool>,
     pub priority:          Option<i32>,
     pub extra_params:      Option<serde_json::Value>,
@@ -184,7 +183,6 @@ impl TryFrom<ModelPayload> for LlmModelRecord {
             model_id:          p.model_id.clone(),
             name:              if p.name.is_empty() { p.model_id } else { p.name },
             strength:          p.strength.as_deref().map(parse_strength).transpose()?,
-            scope:             p.scope.unwrap_or_default(),
             is_default:        p.is_default.unwrap_or(false),
             priority:          p.priority.unwrap_or(100),
             extra_params:      p.extra_params,