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

src/gui_2.py

@@ -2083,15 +2083,6 @@ class App:
    r.destroy()
    if d: self.ui_project_git_dir = d
   imgui.separator()
-  imgui.text("Main Context File")
-  ch, self.ui_project_main_context = imgui.input_text("##main_ctx", self.ui_project_main_context)
-  imgui.same_line()
-  if imgui.button("Browse##ctx"):
-   r = hide_tk_root()
-   p = filedialog.askopenfilename(title="Select Main Context File")
-   r.destroy()
-   if p: self.ui_project_main_context = p
-  imgui.separator()
   imgui.text("Output Dir")
   ch, self.ui_output_dir = imgui.input_text("##out_dir", self.ui_output_dir)
   imgui.same_line()