feat: move autocoder runtime files into .autocoder/ subdirectory

Add centralized path resolution module (autocoder_paths.py) that
consolidates all autocoder-generated file paths behind a dual-path
strategy: check .autocoder/X first, fall back to root-level X for
backward compatibility, default to .autocoder/X for new projects.

Key changes:
- New autocoder_paths.py with dual-path resolution for features.db,
  assistant.db, lock files, settings, prompts dir, and progress cache
- migrate_project_layout() safely moves old-layout projects to new
  layout with SQLite WAL flush and integrity verification
- Updated 22 files to delegate path construction to autocoder_paths
- Reset/delete logic cleans both old and new file locations
- Orphan lock cleanup checks both locations per project
- Migration called automatically at agent start in autonomous_agent_demo.py
- Updated markdown commands/skills to reference .autocoder/prompts/
- CLAUDE.md documentation updated with new project structure

Files at project root that remain unchanged:
- CLAUDE.md (Claude SDK reads from cwd via setting_sources=["project"])
- app_spec.txt root copy (agent templates reference it via cat)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

server/routers/expand_project.py

@@ -136,7 +136,8 @@ async def expand_project_websocket(websocket: WebSocket, project_name: str):
         return
 
     # Verify project has app_spec.txt
-    spec_path = project_dir / "prompts" / "app_spec.txt"
+    from autocoder_paths import get_prompts_dir
+    spec_path = get_prompts_dir(project_dir) / "app_spec.txt"
     if not spec_path.exists():
         await websocket.close(code=4004, reason="Project has no spec. Create spec first.")
         return