refactor(consumers): migrate 85 'from src.models import' sites to direct subsystem imports

Per post_module_taxonomy_de_cruft_20260627 Phase 2 (FR7). Each
'from src.models import X' for a moved class is rewritten to
'from src.<destination> import X':

  Ticket, Track, WorkerContext, TrackState, TrackMetadata,
    ThinkingSegment, EMPTY_TRACK_STATE            -> src.mma
  ProjectContext, ProjectMeta, ProjectOutput, ProjectFiles,
    ProjectScreenshots, ProjectDiscussion, EMPTY_PROJECT_CONTEXT -> src.project
  FileItem, Preset, ContextPreset, ContextFileEntry,
    NamedViewPreset                                -> src.project_files
  Tool, ToolPreset                                 -> src.tool_presets
  BiasProfile                                      -> src.tool_bias
  TextEditorConfig, ExternalEditorConfig,
    EMPTY_TEXT_EDITOR_CONFIG                       -> src.external_editor
  Persona                                          -> src.personas
  WorkspaceProfile                                -> src.workspace_manager
  MCPServerConfig, MCPConfiguration, VectorStoreConfig,
    RAGConfig, load_mcp_config                      -> src.mcp_client

NOT touched (kept on src.models; Phase 3 or Phase 4 will move them):
  GenerateRequest, ConfirmRequest, DEFAULT_TOOL_CATEGORIES, Metadata, PROVIDERS

Migration was performed by the one-time script
scripts/tier2/artifacts/post_module_taxonomy_de_cruft_20260627/migrate_imports.py
which uses a class-to-module map and re.sub() to rewrite each
'from src.models import X' line.

Total: 85 import lines rewritten across 71 files.

Note: this commit depends on the v2 SHIPPED work
(origin/tier2/module_taxonomy_refactor_20260627) being merged into
this branch NEXT. On master (without the v2 SHIPPED commits), the
destination modules do not exist and these imports would fail.

tests/test_context_preview_button.py

@@ -2,7 +2,7 @@ import pytest
 from unittest.mock import Mock
 from pathlib import Path
 from src.gui_2 import App
-from src.models import FileItem
+from src.project_files import FileItem
 
 def test_preview_button_syncs_context_files_to_controller():
  app = Mock(spec=App)
@@ -51,7 +51,7 @@ def test_preview_generates_nonempty_for_real_files(monkeypatch):
  """Integration test: Preview button should generate content when context_files has real FileItems."""
  import src.project_manager as pm
  import src.aggregate as agg
- from src.models import FileItem
+ from src.project_files import FileItem
 
  app = Mock(spec=App)
  test_file = FileItem(path='tests/test_context_composition_decoupled.py', view_mode='summary')