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  _interactive_model_selection (superseded by prompt_toolkit picker)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_tips.py

@@ -1,7 +1,7 @@
 """Tests for hermes_cli/tips.py — random tip display at session start."""
 
 import pytest
-from hermes_cli.tips import TIPS, get_random_tip, get_tip_count
+from hermes_cli.tips import TIPS, get_random_tip
 
 
 class TestTipsCorpus:
@@ -54,11 +54,6 @@ class TestGetRandomTip:
         assert len(seen) >= 10, f"Only got {len(seen)} unique tips in 50 draws"
 
 
-class TestGetTipCount:
-    def test_matches_corpus_length(self):
-        assert get_tip_count() == len(TIPS)
-
-
 class TestTipIntegrationInCLI:
     """Test that the tip display code in cli.py works correctly."""