…t symlink

On Windows, ``os.symlink`` requires Developer Mode or admin privileges;
without them every symlink-touching test raises
``OSError: [WinError 1314] (client does not hold the required privilege)``
during fixture setup, blowing up otherwise-unrelated tests in the same
class. CI is Linux-only so this regression is invisible there, but it
makes the suite unrunnable for contributors on a stock Windows shell.

Add a ``requires_symlinks`` pytest marker. ``conftest.py`` runs a one-shot
``Path.symlink_to`` probe at import time and ``pytest_collection_modifyitems``
auto-skips marked tests when the probe fails (mirrors the existing
``ollama`` skip pattern). Mark the five sites that create real symlinks:

- ``tests/test_runtime_paths.py``: ``test_falls_back_when_xdg_is_symlink``,
  ``test_refuses_existing_symlink``
- ``tests/test_context_settings.py``: ``test_symlink_target_is_treated_as_host_write``
- ``tests/test_context_projects.py``: ``test_discover_symlink_dedup``
- ``tests/test_context_install.py``: ``test_install_skips_symlinks_in_source``
- ``tests/test_web_routes.py``: entire ``TestFsList`` class (the
  ``fs_tree`` fixture creates symlinks, so all 14 tests in the class
  inherit the requirement)

Verified locally:
- macOS (probe=True): 19/19 marked tests PASS — no behavior change on
  POSIX or on Windows + Developer Mode.
- Probe forced to False: 19/19 SKIPPED with a precise reason string —
  matches the locked-down-Windows experience.

Co-Authored-By: Claude <[email protected]>

Review feedback on PR #633: the probe in ``_can_create_symlink`` only
created a file-to-file symlink, but ``TestFsList.fs_tree`` uses
``symlink_to(..., target_is_directory=True)``. Historically Windows
treated directory symlinks as a separate privilege class from file
symlinks (``SeCreateSymbolicLinkPrivilege`` covers both today, but
hardened or older configurations could split them). On such a host the
file-only probe would pass and the 14-test ``TestFsList`` class would
still hard-fail in fixture setup — exactly the regression this PR is
meant to prevent.

Also documents the ``--basetemp`` caveat: the probe runs in
``tempfile.gettempdir()``, so users with ``--basetemp`` pointed at a
filesystem with different symlink semantics (FAT32, some network mounts)
may still see marked tests fail despite the probe passing.

Co-Authored-By: Claude <[email protected]>