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/researcher/meta.json

@@ -14,7 +14,6 @@
   },
   "instructions": "Pass a specific research question; optionally hint at depth (how many sources) or a time horizon. Optionally specify an output file/dir in the prompt to write the report outside the default `data/research/`. Returns a path + one-line summary, also saved to the scratchpad.",
   "type": "task",
-  "scope": "general",
   "strength": "average",
   "icon": "icon.png"
 }