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/project_manager.py

@@ -100,7 +100,7 @@ def default_project(name: str = "unnamed") -> dict[str, Any]:
  [C: tests/test_deepseek_infra.py:test_default_project_includes_reasoning_role, tests/test_discussion_takes.py:TestDiscussionTakes.setUp, tests/test_history_management.py:test_history_persistence_across_turns, tests/test_history_management.py:test_save_separation, tests/test_project_manager_modes.py:test_default_project_execution_mode, tests/test_project_manager_modes.py:test_load_save_execution_mode, tests/test_project_serialization.py:TestProjectSerialization.test_default_roles_include_context, tests/test_project_serialization.py:TestProjectSerialization.test_fileitem_roundtrip]
  """
  return {
-  "project":     {"name": name, "git_dir": "", "system_prompt": "", "main_context": "", "execution_mode": "native"},
+  "project":     {"name": name, "git_dir": "", "system_prompt": "", "execution_mode": "native"},
   "output":      {"output_dir": "./md_gen"},
   "files":       {"base_dir": ".", "paths": [], "tier_assignments": {}},
   "screenshots": {"base_dir": ".", "paths": []},