Fix zsh completion to respect ZDOTDIR env var

[Fridayai700]

Type of changes

• Bug fix

AI?

• AI was used to generate this PR

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate (see note about typos above).
• I've added tests for new code.
• I accept that @willmcgugan may be pedantic in the code review.

Description

Fixes #171

When installing zsh autocompletion, install_zsh() hardcoded Path.home() for both .zshrc and .zfunc/ paths. This ignores the ZDOTDIR environment variable, which zsh uses to locate user startup files.

The fix:

• Check os.environ.get("ZDOTDIR") and use it as the base directory when set
• Fall back to Path.home() when ZDOTDIR is not set (preserving existing behavior)
• Use $ZDOTDIR/.zfunc in the fpath line when ZDOTDIR is set (zsh expands this at startup), ~/.zfunc otherwise

Test added: test_completion_install_zsh_zdotdir — creates a temp directory as ZDOTDIR, runs completion install, verifies .zshrc and .zfunc/ are created in the ZDOTDIR location.

All existing completion install tests continue to pass.