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  _interactive_model_selection (superseded by prompt_toolkit picker)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_plugin_cli_registration.py

@@ -12,7 +12,7 @@ import argparse
 import os
 import sys
 from pathlib import Path
-from unittest.mock import MagicMock, patch
+from unittest.mock import MagicMock
 
 import pytest
 
@@ -20,7 +20,6 @@ from hermes_cli.plugins import (
     PluginContext,
     PluginManager,
     PluginManifest,
-    get_plugin_cli_commands,
 )
 
 
@@ -64,18 +63,6 @@ class TestRegisterCliCommand:
         assert mgr._cli_commands["nocb"]["handler_fn"] is None
 
 
-class TestGetPluginCliCommands:
-    def test_returns_dict(self):
-        mgr = PluginManager()
-        mgr._cli_commands["foo"] = {"name": "foo", "help": "bar"}
-        with patch("hermes_cli.plugins.get_plugin_manager", return_value=mgr):
-            cmds = get_plugin_cli_commands()
-        assert cmds == {"foo": {"name": "foo", "help": "bar"}}
-        # Top-level is a copy — adding to result doesn't affect manager
-        cmds["new"] = {"name": "new"}
-        assert "new" not in mgr._cli_commands
-
-
 # ── Memory plugin CLI discovery ───────────────────────────────────────────