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  _interactive_model_selection (superseded by prompt_toolkit picker)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_skills_tool.py

@@ -13,11 +13,9 @@ from tools.skills_tool import (
     _parse_frontmatter,
     _parse_tags,
     _get_category_from_path,
-    _estimate_tokens,
     _find_all_skills,
     skill_matches_platform,
     skills_list,
-    skills_categories,
     skill_view,
     MAX_DESCRIPTION_LENGTH,
 )
@@ -190,18 +188,6 @@ class TestGetCategoryFromPath:
             assert _get_category_from_path(skill_md) is None
 
 
-# ---------------------------------------------------------------------------
-# _estimate_tokens
-# ---------------------------------------------------------------------------
-
-
-class TestEstimateTokens:
-    def test_estimate(self):
-        assert _estimate_tokens("1234") == 1
-        assert _estimate_tokens("12345678") == 2
-        assert _estimate_tokens("") == 0
-
-
 # ---------------------------------------------------------------------------
 # _find_all_skills
 # ---------------------------------------------------------------------------
@@ -544,32 +530,6 @@ class TestSkillViewSecureSetupOnLoad:
         assert result["content"].startswith("---")
 
 
-# ---------------------------------------------------------------------------
-# skills_categories
-# ---------------------------------------------------------------------------
-
-
-class TestSkillsCategories:
-    def test_lists_categories(self, tmp_path):
-        with patch("tools.skills_tool.SKILLS_DIR", tmp_path):
-            _make_skill(tmp_path, "s1", category="devops")
-            _make_skill(tmp_path, "s2", category="mlops")
-            raw = skills_categories()
-        result = json.loads(raw)
-        assert result["success"] is True
-        names = {c["name"] for c in result["categories"]}
-        assert "devops" in names
-        assert "mlops" in names
-
-    def test_empty_skills_dir(self, tmp_path):
-        skills_dir = tmp_path / "skills"
-        with patch("tools.skills_tool.SKILLS_DIR", skills_dir):
-            raw = skills_categories()
-        result = json.loads(raw)
-        assert result["success"] is True
-        assert result["categories"] == []
-
-
 # ---------------------------------------------------------------------------
 # skill_matches_platform
 # ---------------------------------------------------------------------------