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  _interactive_model_selection (superseded by prompt_toolkit picker)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/tools/test_vision_tools.py

@@ -21,7 +21,6 @@ from tools.vision_tools import (
     _RESIZE_TARGET_BYTES,
     vision_analyze_tool,
     check_vision_requirements,
-    get_debug_session_info,
 )
 
 
@@ -441,7 +440,7 @@ class TestVisionSafetyGuards:
 
 
 # ---------------------------------------------------------------------------
-# check_vision_requirements & get_debug_session_info
+# check_vision_requirements
 # ---------------------------------------------------------------------------
 
 
@@ -466,14 +465,6 @@ class TestVisionRequirements:
 
         assert check_vision_requirements() is True
 
-    def test_debug_session_info_returns_dict(self):
-        info = get_debug_session_info()
-        assert isinstance(info, dict)
-        # DebugSession.get_session_info() returns these keys
-        assert "enabled" in info
-        assert "session_id" in info
-        assert "total_calls" in info
-
 
 # ---------------------------------------------------------------------------
 # Integration: registry entry