Summary

• limit filename completions to string literals, shell/magic contexts, and explicit path-looking tokens
• keep direct path completion for prefixes such as ./, ../, ~, absolute paths, and Windows drive paths
• include name = !command as shell syntax by reusing IPython's system-assignment tokenizer

Fixes #14501.

Local checks

• python -m pytest tests/test_completer.py::TestCompleter::test_file_completions_only_in_path_contexts tests/test_completer.py::TestCompleter::test_local_file_completions -q
• python -m pytest tests/test_completer.py -q
• python -m ruff check IPython/core/completer.py
• python -m compileall -q IPython/core/completer.py tests/test_completer.py
• git diff --check