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/agents.rs

@@ -52,7 +52,7 @@ pub async fn get(
     meta.localize(&locale);
     let prompt = skald_core::agents::load_prompt(&id)?;
     let all    = skald.llm_manager().list_models_info().await;
-    let models = sort_models_for_agent(all, meta.scope.as_deref(), meta.strength);
+    let models = sort_models_for_agent(all, meta.strength);
     Ok(Json(AgentDetail { meta, prompt, models }))
 }
 

src/frontend/api/config.rs

@@ -65,6 +65,15 @@ pub async fn list_properties(
             name: skald_core::i18n::native_language_name(code),
         })
         .collect::<Vec<_>>();
+    // Configured LLM models, keyed by `name` (the resolution key LlmManager
+    // uses), labelled with the provider for disambiguation.
+    let llm_models = skald.llm_manager().list_models_info().await
+        .into_iter()
+        .map(|m| SelectOption {
+            id:   m.name.clone(),
+            name: format!("{} ({})", m.name, m.provider_name),
+        })
+        .collect::<Vec<_>>();
 
     let mut sets = Vec::with_capacity(skald.config_properties().len());
     for set in skald.config_properties() {
@@ -78,6 +87,7 @@ pub async fn list_properties(
                 PropertyType::String        => ("string", None),
                 PropertyType::SecurityGroup => ("security_group", Some(security_groups.clone())),
                 PropertyType::Locale        => ("locale", Some(locales.clone())),
+                PropertyType::LlmModel      => ("llm_model", Some(llm_models.clone())),
             };
             props.push(PropertyView {
                 key:           prop.key.clone(),

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,