fix: avoid literal {{WORKSPACE_NAME}} directories in built images

When a manifest mount target uses ~/{{WORKSPACE_NAME}}, the Dockerfile was
building a literal directory named {{WORKSPACE_NAME}} into the image and
creating a broken /workspace symlink. The runtime mount then created the
correct repo-named folder alongside the placeholder folder.

- Only create static mount target directories in the Dockerfile; skip any
  target containing {{WORKSPACE_NAME}}
- Only create the /workspace compatibility symlink at image-build time when
  the workspace name is known; otherwise let the entrypoint create it from
  the WORKSPACE_NAME environment variable
- Update unit tests to cover both build-time workspace names and runtime
  placeholders

Quality gates:
- pytest tests/unit: 211 passed
- ruff: clean on changed files
- mypy: clean on changed files

apps/api/tests/unit/.pi-map.index.md

@@ -2,7 +2,7 @@
 dir: apps/api/tests/unit
 
 ## role
-Unit test suite for the API application covering core services, utilities, and infrastructure components.
+Comprehensive unit test suite for the API backend, covering core services, utilities, configuration, and infrastructure components.
 ## parent
 index: apps/api/tests/.pi-map.index.md
 map: apps/api/tests/.pi-map.md