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/skald-core/src/llm/mod.rs

@@ -17,7 +17,6 @@ pub struct LlmEntry {
     pub model:           String,
     pub model_db_id:     i64,
     pub strength:        Option<LlmStrength>,
-    pub scope:           Vec<String>,
     pub extra_params:    Option<serde_json::Value>,
     /// Max input context window in tokens, if known.
     pub context_length:  Option<i64>,
@@ -96,7 +95,6 @@ pub struct LlmModelInfo {
     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>,