feat: keep new Windows local experts on one absolute subtree root

[bobbanga]

Summary

• New Windows local_shell / filesystem experts store one fully qualified absolute root_dir, with virtual_mode on and root_semantics=subtree. Relative paths, drive-relative paths, and virtual_mode=False are rejected before any directory is created.
• Docker, remote storage, and composite stay on the existing create and permission path. They are not stamped, and a missing explicit_virtual_paths field does not block them. Unknown or malformed backend representations are rejected.
• root_semantics is preserved across ordinary updates. Loading a subtree record fails closed when the installed harness cannot set explicit_virtual_paths.
• The storage-root picker now follows the host. Windows lists every ready drive (C:/, D:/, ...) as its own collapsed row and preselects the home drive. macOS and Linux keep a single / tree. jail_enforced is true only when bubblewrap is actually available, so Windows and macOS no longer claim an OS sandbox.

Dependency

• Requires the harness flag added in feat: convert explicit virtual paths for subtree experts octop-harness#30
• Octop tests imported that harness tree at commit 58cb7b0d044dded11481612de83b972f807f197c (/Users/bob/Code/work/octop-harness/src/octop_harness/__init__.py via PYTHONPATH). They did not import the PyPI octop-harness 1.0.0 wheel.
• This PR can be reviewed against that commit. A subtree expert cannot run until that harness change is what the process imports.

Test plan

Windows picker, verified on a Windows machine. Opening create-from the general expert shows C:/ selected, with ready drives as collapsed rows (C:, D:, E:, F:, H: on that machine). The hint says file access is limited and there is no OS sandbox.

The two groups below go through AgentManager.create / stamp_new_windows_subtree: supported non-local backends are not cut off, and a local string alias cannot skip the new rule.

PYTHONPATH="/Users/bob/Code/work/octop/src:/Users/bob/Code/work/octop-harness/src" \
  /Users/bob/Code/work/octop/.venv/bin/pytest \
  tests/unit/agents/test_windows_subtree_entry.py::test_create_keeps_supported_non_local_backends \
  tests/unit/agents/test_windows_subtree_entry.py::test_create_composite_keeps_the_existing_scoped_workspace \
  tests/unit/agents/test_windows_subtree_entry.py::test_create_normalizes_string_backend_and_rejects_relative_home \
  tests/unit/agents/test_windows_subtree_entry.py::test_create_rejects_uninterpretable_backend_before_mkdir \
  tests/unit/agents/test_windows_root.py::test_stamp_leaves_supported_non_local_backends_alone \
  tests/unit/agents/test_windows_root.py::test_stamp_normalizes_local_backend_strings \
  -vv --tb=short

============================= test session starts ==============================
platform darwin -- Python 3.12.14, pytest-9.1.1, pluggy-1.6.0 -- /Users/bob/Code/work/octop/.venv/bin/python3
cachedir: .pytest_cache
rootdir: /Users/bob/Code/work/octop
configfile: pyproject.toml
plugins: cov-7.1.0, anyio-4.14.1, xdist-3.8.0, asyncio-1.4.0, langsmith-0.11.2, testmon-2.2.0
asyncio: mode=Mode.AUTO, debug=False, asyncio_default_fixture_loop_scope=None, asyncio_default_test_loop_scope=function
collecting ... collected 16 items

tests/unit/agents/test_windows_subtree_entry.py::test_create_keeps_supported_non_local_backends[backend0] PASSED [  6%]
tests/unit/agents/test_windows_subtree_entry.py::test_create_keeps_supported_non_local_backends[docker] PASSED [ 12%]
tests/unit/agents/test_windows_subtree_entry.py::test_create_keeps_supported_non_local_backends[backend2] PASSED [ 18%]
tests/unit/agents/test_windows_subtree_entry.py::test_create_keeps_supported_non_local_backends[backend3] PASSED [ 25%]
tests/unit/agents/test_windows_subtree_entry.py::test_create_composite_keeps_the_existing_scoped_workspace PASSED [ 31%]
tests/unit/agents/test_windows_subtree_entry.py::test_create_normalizes_string_backend_and_rejects_relative_home PASSED [ 37%]
tests/unit/agents/test_windows_subtree_entry.py::test_create_rejects_uninterpretable_backend_before_mkdir[nope] PASSED [ 43%]
tests/unit/agents/test_windows_subtree_entry.py::test_create_rejects_uninterpretable_backend_before_mkdir[backend1] PASSED [ 50%]
tests/unit/agents/test_windows_subtree_entry.py::test_create_rejects_uninterpretable_backend_before_mkdir[backend2] PASSED [ 56%]
tests/unit/agents/test_windows_subtree_entry.py::test_create_rejects_uninterpretable_backend_before_mkdir[backend3] PASSED [ 62%]
tests/unit/agents/test_windows_root.py::test_stamp_leaves_supported_non_local_backends_alone[docker] PASSED [ 68%]
tests/unit/agents/test_windows_root.py::test_stamp_leaves_supported_non_local_backends_alone[backend1] PASSED [ 75%]
tests/unit/agents/test_windows_root.py::test_stamp_leaves_supported_non_local_backends_alone[backend2] PASSED [ 81%]
tests/unit/agents/test_windows_root.py::test_stamp_leaves_supported_non_local_backends_alone[backend3] PASSED [ 87%]
tests/unit/agents/test_windows_root.py::test_stamp_leaves_supported_non_local_backends_alone[opensandbox] PASSED [ 93%]
tests/unit/agents/test_windows_root.py::test_stamp_normalizes_local_backend_strings PASSED [100%]

============================== 16 passed in 1.37s ==============================

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