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_execution_engine.py

@@ -1,4 +1,4 @@
-from src.models import Ticket
+from src.mma import Ticket
 from src.dag_engine import TrackDAG, ExecutionEngine
 
 def test_execution_engine_basic_flow():