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

gencpp_manual_slop_template.toml

@@ -3,7 +3,6 @@
 [project]
 name = "gencpp"
 git_dir = ".git"
-main_context = "GenCpp: A C++ library for agenic code generation and analysis."
 word_wrap = true
 
 [paths]