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  _interactive_model_selection (superseded by prompt_toolkit picker)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/hermes_cli/test_skin_engine.py

@@ -40,13 +40,6 @@ class TestSkinConfig:
         assert skin.get_branding("agent_name") == "Hermes Agent"
         assert skin.get_branding("nonexistent", "fallback") == "fallback"
 
-    def test_get_spinner_list_empty_for_default(self):
-        from hermes_cli.skin_engine import load_skin
-        skin = load_skin("default")
-        # Default skin has no custom spinner config
-        assert skin.get_spinner_list("waiting_faces") == []
-        assert skin.get_spinner_list("thinking_verbs") == []
-
     def test_get_spinner_wings_empty_for_default(self):
         from hermes_cli.skin_engine import load_skin
         skin = load_skin("default")
@@ -68,9 +61,6 @@ class TestBuiltinSkins:
     def test_ares_has_spinner_customization(self):
         from hermes_cli.skin_engine import load_skin
         skin = load_skin("ares")
-        assert len(skin.get_spinner_list("waiting_faces")) > 0
-        assert len(skin.get_spinner_list("thinking_faces")) > 0
-        assert len(skin.get_spinner_list("thinking_verbs")) > 0
         wings = skin.get_spinner_wings()
         assert len(wings) > 0
         assert isinstance(wings[0], tuple)