`bst shell` is giving a bug trace when using `--mount`

I'm trying to work out good ways to add some tests to a bst project

I thought about using some of the software inside a shell, this works for basic `command --help` tests.

```
toolchain-2026-06-03 src # bst shell components/python3-mkdocs.bst -- mkdocs --help
[--:--:--][ ][ main:core activity ] START Loading elements
[00:00:00][ ][ main:core activity ] SUCCESS Loading elements
[--:--:--][ ][ main:core activity ] START Resolving elements
[00:00:00][ ][ main:core activity ] SUCCESS Resolving elements
[--:--:--][ ][ main:core activity ] START Initializing remote caches
[00:00:02][ ][ main:core activity ] SUCCESS Initializing remote caches
[--:--:--][ ][ main:core activity ] START Query cache
[00:00:00][ ][ main:core activity ] SUCCESS Query cache
[--:--:--][d37eff3b][ main:components/python3-mkdocs.bst ] START Staging dependencies
[00:00:00][d37eff3b][ main:components/python3-mkdocs.bst ] SUCCESS Staging dependencies
[--:--:--][d37eff3b][ main:components/python3-mkdocs.bst ] START Integrating sandbox
[00:00:00][d37eff3b][ main:components/python3-mkdocs.bst ] SUCCESS Integrating sandbox
[--:--:--][d37eff3b][ main:components/python3-mkdocs.bst ] STATUS Running command

mkdocs --help

Usage: mkdocs [OPTIONS] COMMAND [ARGS]...

MkDocs - Project documentation with Markdown.

Options:
-V, --version Show the version and exit.
-q, --quiet Silence warnings
-v, --verbose Enable verbose output
--color / --no-color Force enable or disable color and wrapping for the output. Default is auto-detect.
-h, --help Show this message and exit.

Commands:
build Build the MkDocs documentation.
get-deps Show required PyPI packages inferred from plugins in mkdocs.yml.
gh-deploy Deploy your documentation to GitHub Pages.
new Create a new MkDocs project.
serve Run the builtin development server.
dcs-toolchain-2026-06-03 src #
```

but when i try to add a example project to use i need to use `--mount` which triggers a bug for me

```
toolchain-2026-06-03 src # ls /src/tests/
config.yml index.md

toolchain-2026-06-03 src # bst shell components/python3-mkdocs.bst --mount /src/tests/ -- mkdocs build
[--:--:--][ ][ main:core activity ] START Loading elements
[00:00:00][ ][ main:core activity ] SUCCESS Loading elements
[--:--:--][ ][ main:core activity ] START Resolving elements
[00:00:00][ ][ main:core activity ] SUCCESS Resolving elements
[--:--:--][ ][ main:core activity ] START Initializing remote caches
[00:00:02][ ][ main:core activity ] SUCCESS Initializing remote caches
[--:--:--][ ][ main:core activity ] START Query cache
[00:00:00][ ][ main:core activity ] SUCCESS Query cache
[--:--:--][d37eff3b][ main:components/python3-mkdocs.bst ] START Staging dependencies
[00:00:00][d37eff3b][ main:components/python3-mkdocs.bst ] SUCCESS Staging dependencies
[--:--:--][d37eff3b][ main:components/python3-mkdocs.bst ] START Integrating sandbox
[00:00:00][d37eff3b][ main:components/python3-mkdocs.bst ] SUCCESS Integrating sandbox
[--:--:--][ ][ main:core activity ] BUG The directories marked in the sandbox must be absolute paths

Traceback (most recent call last):
File "/usr/sbin/bst", line 8, in
sys.exit(cli())
~~~^^
File "/usr/lib/python3.13/site-packages/click/core.py", line 1442, in __call__
return self.main(*args, **kwargs)
~~~~~~~~~^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.13/site-packages/buildstream/_frontend/cli.py", line 282, in override_main
original_main(self, args=args, prog_name=prog_name, complete_var=None, standalone_mode=standalone_mode, **extra)
~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.13/site-packages/click/core.py", line 1363, in main
rv = self.invoke(ctx)
File "/usr/lib/python3.13/site-packages/click/core.py", line 1830, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
File "/usr/lib/python3.13/site-packages/click/core.py", line 1226, in invoke
return ctx.invoke(self.callback, **ctx.params)
~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.13/site-packages/click/core.py", line 794, in invoke
return callback(*args, **kwargs)
File "/usr/lib/python3.13/site-packages/click/decorators.py", line 46, in new_func
return f(get_current_context().obj, *args, **kwargs)
File "/usr/lib/python3.13/site-packages/buildstream/_frontend/cli.py", line 774, in shell
exitcode = app.stream.shell(
target,
...<9 lines>...
ignore_project_source_remotes=ignore_project_source_remotes,
)
File "/usr/lib/python3.13/site-packages/buildstream/_stream.py", line 360, in shell
return element._shell(
~~~~~~~~~~~~~~^
scope, mounts=mounts, isolate=isolate, prompt=prompt(element), command=command, usebuildtree=usebuildtree
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
)
^
File "/usr/lib/python3.13/site-packages/buildstream/element.py", line 2099, in _shell
sandbox.mark_directory(mount.path)
~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^
File "/usr/lib/python3.13/site-packages/buildstream/sandbox/sandbox.py", line 168, in mark_directory
assert directory.startswith("/"), "The directories marked in the sandbox must be absolute paths"
~~~~~~~~~~~~~~~~~~~~^^^^^
AssertionError: The directories marked in the sandbox must be absolute paths
```

Reproduce the failure with `bst shell components/python3-mkdocs.bst --mount /src/tests/ -- mkdocs build`. Start at `buildstream/element.py` in `_shell`, then follow the mount handling into `buildstream/sandbox/sandbox.py` at `mark_directory`; compare this path with the CLI `shell` entry point in `buildstream/_frontend/cli.py`. Done means the reported mount command runs without the assertion or bug trace.

Written by the indexing model from the issue text.