refactor: remove dead code — 1,784 lines across 77 files (#9180)

Deep scan with vulture, pyflakes, and manual cross-referencing identified:
- 41 dead functions/methods (zero callers in production)
- 7 production-dead functions (only test callers, tests deleted)
- 5 dead constants/variables
- ~35 unused imports across agent/, hermes_cli/, tools/, gateway/

Categories of dead code removed:
- Refactoring leftovers: _set_default_model, _setup_copilot_reasoning_selection,
  rebuild_lookups, clear_session_context, get_logs_dir, clear_session
- Unused API surface: search_models_dev, get_pricing, skills_categories,
  get_read_files_summary, clear_read_tracker, menu_labels, get_spinner_list
- Dead compatibility wrappers: schedule_cronjob, list_cronjobs, remove_cronjob
- Stale debug helpers: get_debug_session_info copies in 4 tool files
  (centralized version in debug_helpers.py already exists)
- Dead gateway methods: send_emote, send_notice (matrix), send_reaction
  (bluebubbles), _normalize_inbound_text (feishu), fetch_room_history
  (matrix), _start_typing_indicator (signal), parse_feishu_post_content
- Dead constants: NOUS_API_BASE_URL, SKILLS_TOOL_DESCRIPTION,
  FILE_TOOLS, VALID_ASPECT_RATIOS, MEMORY_DIR
- Unused UI code: _interactive_provider_selection,
  _interactive_model_selection (superseded by prompt_toolkit picker)

Test suite verified: 609 tests covering affected files all pass.
Tests for removed functions deleted. Tests using removed utilities
(clear_read_tracker, MEMORY_DIR) updated to use internal APIs directly.

tools/mixture_of_agents_tool.py

@@ -416,29 +416,6 @@ def check_moa_requirements() -> bool:
     return check_openrouter_api_key()
 
 
-def get_debug_session_info() -> Dict[str, Any]:
-    """
-    Get information about the current debug session.
-    
-    Returns:
-        Dict[str, Any]: Dictionary containing debug session information
-    """
-    return _debug.get_session_info()
-
-
-def get_available_models() -> Dict[str, List[str]]:
-    """
-    Get information about available models for MoA processing.
-    
-    Returns:
-        Dict[str, List[str]]: Dictionary with reference and aggregator models
-    """
-    return {
-        "reference_models": REFERENCE_MODELS,
-        "aggregator_models": [AGGREGATOR_MODEL],
-        "supported_models": REFERENCE_MODELS + [AGGREGATOR_MODEL]
-    }
-
 
 def get_moa_configuration() -> Dict[str, Any]:
     """