[skip ci]

Co-authored-by: Copilot <[email protected]>

[skip ci]

Replace shell=True with list-based subprocess calls for all git.py
functions that interpolate user-controlled values (tag names, messages,
file paths, git references). This prevents shell injection attacks where
malicious values in pyproject.toml could execute arbitrary commands
during CI/CD runs of 'cz bump'.

Changes:
- cmd.run() now accepts str | Sequence[str]; lists use shell=False
- git.tag() uses list args (fixes primary attack vector)
- git.add() uses list args
- git.commit() uses list args + env= for GIT_COMMITTER_DATE
- git.tag_exist/is_signed_tag/get_tag_message use list args
- git.get_filenames_in_commit() uses list args
- git.get_tags() uses list args
- git._get_log_as_str_list() uses list args

Closes #1918

Co-authored-by: Copilot <[email protected]>