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/db/mod.rs

@@ -206,7 +206,6 @@ async fn create_registry_tables(pool: &SqlitePool) -> Result<()> {
             model_id          TEXT    NOT NULL,
             name              TEXT    NOT NULL UNIQUE,
             strength          TEXT,
-            scope             TEXT    NOT NULL DEFAULT '[]',
             is_default        INTEGER NOT NULL DEFAULT 0,
             priority          INTEGER NOT NULL DEFAULT 100,
             extra_params      TEXT,