Complete snapshot refs in shell completions

[vku2018]

Closes #94.

Summary

• add a hidden quicksave __complete-refs helper that quietly prints snapshot numbers, ids, and names for shell completion
• wire bash, zsh, fish, and PowerShell completions to suggest refs at snapshot-ref argument positions while preserving file fallback elsewhere
• keep the hidden helper out of the normal subcommand list and cover the helper/completion output with tests

Validation

• python -m pytest -q tests/test_cli.py::test_completion_bash_lists_subcommands tests/test_cli.py::test_completion_zsh_wires_compdef tests/test_cli.py::test_completion_fish_lists_subcommands tests/test_cli.py::test_completion_powershell_registers_completer tests/test_cli.py::test_complete_refs_prints_snapshot_numbers_ids_and_names tests/test_cli.py::test_complete_refs_outside_project_is_quiet
• python -m compileall -q quicksave tests
• git diff --check
• generated PowerShell completer registers successfully via Invoke-Expression
• python -m pytest -q -k "not no_color_env_strips_styling" (290 passed, 1 deselected)

Note

• Full python -m pytest -q on this Windows runner reports one existing platform-sensitive failure in tests/test_cli.py::test_no_color_env_strips_styling; the same test fails when run standalone because captured output does not include ANSI styling before NO_COLOR is set.