core/tools: send a deleted path to the system trash

`delete_path` is the only destructive file entry point the agent has, and it
carries `ToolTag.YOLO` — under `--yes` / `/yolo` it runs with no confirmation at
all. A delete is now a move into the desktop's own trash (freedesktop trash on
Linux, Finder trash on macOS, Recycle Bin on Windows) through `send2trash`, so
the human can put the file back with the file manager they already have. The new
`permanent=true` argument is the old behaviour, chosen on purpose.

A trashing that fails is reported, never downgraded to a deletion: the human was
told the path is recoverable, so a volume with no writable trash directory
leaves the file where it is and the error names `permanent=true` as the way to
delete it anyway. A broken install (the library is a required dependency) is
reported the same way.

Scratch paths under the OS temporary directory are still deleted for good —
nobody restores them, and a trash on the `/tmp` volume would only collect them.
The rule now lives in `tools/trash.py` (`under_system_temp`) and the temporary
workspace cleanup uses it too, instead of keeping a second copy of the check.

typings/send2trash/__init__.pyi

@@ -0,0 +1,9 @@
+# Minimal stubs for ``send2trash`` (a required dependency).
+# Only the surface ``plyngent.tools.trash`` uses is declared; the real package is
+# pure Python over ctypes (freedesktop trash spec / Finder trash / Recycle Bin).
+
+from collections.abc import Iterable
+
+class TrashPermissionError(PermissionError): ...
+
+def send2trash(paths: str | Iterable[str]) -> None: ...