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.

agents/code-explorer/meta.json

@@ -14,7 +14,6 @@
   },
   "instructions": "Give it a concrete question or area to investigate (a bug, a module, an architecture concern). It writes a Markdown report to data/explorer/ and returns a summary. It never edits code or plans work.",
   "type": "task",
-  "scope": "reasoning",
   "strength": "high",
   "icon": "icon.png"
 }