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.

web/components/config-page.js

@@ -11,6 +11,7 @@ function _configSetSlug(name) {
   const slugs = {
     'Interface': 'interface',
     'TIC Agent': 'tic_agent',
+    'Compaction': 'compaction',
   };
   return slugs[name] ?? null;
 }
@@ -193,6 +194,20 @@ export class ConfigPage extends LightElement {
         </select>`;
     }
 
+    if (prop.property_type === 'llm_model') {
+      // Configured LLM models, by name. Nullable: the empty choice means
+      // "auto-select" (the backend's own resolution order applies).
+      const models = prop.options ?? [];
+      return html`
+        <select class="form-select form-select-sm config-input"
+                .value=${val}
+                @change=${e => this._setValue(prop.key, e.target.value)}>
+          <option value="">— ${t('config.llm_model.auto')} —</option>
+          ${models.map(m => html`
+            <option value=${m.id} ?selected=${val === m.id}>${m.name}</option>`)}
+        </select>`;
+    }
+
     return html`
       <input type="text"
              class="form-control form-control-sm config-input"