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  _interactive_model_selection (superseded by prompt_toolkit picker)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.

tests/integration/test_web_tools.py

@@ -34,7 +34,6 @@ from tools.web_tools import (
     check_firecrawl_api_key,
     check_web_api_key,
     check_auxiliary_model,
-    get_debug_session_info,
     _get_backend,
 )
 
@@ -138,12 +137,6 @@ class WebToolsTester:
         else:
             self.log_result("Auxiliary LLM", "passed", "Found")
         
-        # Check debug mode
-        debug_info = get_debug_session_info()
-        if debug_info["enabled"]:
-            print_info(f"Debug mode enabled - Session: {debug_info['session_id']}")
-            print_info(f"Debug log: {debug_info['log_path']}")
-        
         return True
     
     def test_web_search(self) -> List[str]:
@@ -585,7 +578,6 @@ class WebToolsTester:
                 "firecrawl_api_key": check_firecrawl_api_key(),
                 "parallel_api_key": bool(os.getenv("PARALLEL_API_KEY")),
                 "auxiliary_model": check_auxiliary_model(),
-                "debug_mode": get_debug_session_info()["enabled"]
             }
         }