feat(gui): Remove ui_summary_only global toggle

The ui_summary_only global aggregation toggle was redundant with per-file flags
(auto_aggregate, force_full). Removed:
- Checkbox from Projects panel (gui_2.py)
- State variable and project load/save (app_controller.py)

Per-file flags remain the intended mechanism for controlling aggregation.

Tests added to verify removal and per-file flag functionality.

conductor/tracks/discussion_hub_panel_reorganization_20260322/plan.md

@@ -3,8 +3,8 @@
 ## Phase 1: Cleanup & Project Settings Rename
 Focus: Remove redundant ui_summary_only, rename Context Hub, establish project-level vs discussion-level separation
 
-- [ ] Task: Audit current ui_summary_only usages and document behavior to deprecate
-- [ ] Task: Remove ui_summary_only checkbox from _render_projects_panel (gui_2.py)
+- [x] Task: Audit current ui_summary_only usages and document behavior to deprecate [f6fe3ba] (embedded audit)
+- [~] Task: Remove ui_summary_only checkbox from _render_projects_panel (gui_2.py)
 - [ ] Task: Rename Context Hub to "Project Settings" in _gui_func tab bar
 - [ ] Task: Remove Context Presets tab from Project Settings (Context Hub)
 - [ ] Task: Update references in show_windows dict and any help text