fix: strip anchored glob patterns on Windows to avoid NotImplementedError

[icn5381]

Issue

Fixes #5679
Fixes #5675

On Windows, a pattern starting with / is not absolute per os.path.isabs() (no drive letter), but pathlib.Path.glob() treats it as an "anchored" pattern and raises NotImplementedError: Non-relative patterns are unsupported. This crashes /add and /read-only when the user passes a forward-slash path like /add /some/file.py.

Fix

• Add _safe_glob_pattern() helper: on Windows (os.name == "nt"), strips the leading / from a pattern so Path.glob() treats it as relative to the glob root (which is what the user intended). No-op on POSIX.
• Apply it at both Path.glob() call sites in commands.py (glob_filtered_to_repo ~line 774, read-only file collection ~line 1361).
• Add NotImplementedError to the existing exception handler in glob_filtered_to_repo as a fallback for any other anchored-pattern shapes.

Testing

4 new unit tests in TestSafeGlobPattern cover:

• POSIX passthrough (unchanged)
• Windows strips /foo/bar → foo/bar
• Windows preserves non-anchored patterns (*.py, src/file.py)
• Windows preserves drive paths (C:/drive/file.py)

Note: the full test suite could not be run locally because Python 3.14 removed audioop, which pydub (an aider dependency) requires. The helper function logic was verified independently with mocked os.name on both platforms.

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