fix: remove stale {{WORKSPACE_NAME}} directory from container home

Older cached images still contain a literal /home/user/{{WORKSPACE_NAME}}
directory baked in by the previous Dockerfile generation. Even though new
images no longer create it, existing images leave the placeholder folder
alongside the real repo-named mount.

- Add entrypoint cleanup that removes /{{WORKSPACE_NAME}} if it
  exists before creating the real workspace target and /workspace symlink
- Update unit tests to assert the stale placeholder removal

Quality gates:
- pytest tests/unit: 212 passed
- ruff: clean on changed files
- mypy: clean on changed files

openspec/changes/.pi-map.index.md

@@ -2,7 +2,7 @@
 dir: openspec/changes
 
 ## role
-Provides change-tracking and diffing utilities for OpenAPI specification evolution and version comparison.
+Manages change tracking, versioning, and history for OpenAPI specification modifications
 ## parent
 index: openspec/.pi-map.index.md
 map: openspec/.pi-map.md