refactor: remove dead main_context field from Project Settings

The main_context field in Project Settings was stored but never used.
Nothing reads it to inject into AI context. System Prompt in AI Settings
already serves this purpose.

Removed:
- app_controller.py: ui_project_main_context state variable and all refs
- gui_2.py: Main Context File UI section from Projects panel
- project_manager.py: main_context from default_project()
- project.toml, manual_slop.toml, gencpp_manual_slop_template.toml: main_context entries

manual_slop.toml

@@ -6,7 +6,6 @@ word_wrap = true
 summary_only = false
 auto_scroll_comms = true
 auto_scroll_tool_calls = true
-main_context = ""
 
 [output]
 output_dir = "./md_gen"