fix: surface real workspace creation errors instead of generic 409

The create-workspace endpoints caught every exception and returned 409
"Workspace name already exists", hiding the actual failure (e.g. git
clone errors, remote URL problems, or filesystem issues).

- Distinguish ValueError -> 400, unexpected exceptions -> 500 with detail
- Preserve HTTPException re-raise for existing FastAPI error paths

Quality gates:
- pytest tests/unit: 219 passed
- mypy: clean on changed files

apps/.pi-map.index.md

@@ -2,7 +2,7 @@
 dir: apps
 
 ## role
-Contains the main executable applications and entry points for the project
+Contains the main executable application entry points and top-level app configurations for the project.
 ## parent
 index: ./.pi-map.index.md
 map: ./.pi-map.md