Automatically install shell tab completion after install

Summary

The installer currently prints shell completion setup guidance, but users still need to manually run the appropriate wtk completion <shell> command and place the generated script in the right location. It would be nicer if installing wtk also enabled tab completion automatically when possible.

Current behavior

• scripts/install.sh installs the wtk binary and verifies wtk --version.
• After installation, it prints completion setup guidance.
• Users must manually configure completion for bash, zsh, fish, or PowerShell.

Proposed behavior

After a successful install, the installer should detect the user's shell and install/update the matching completion script automatically when it can do so safely.

Potential behavior:

• zsh: write _wtk to an appropriate user-writable directory in fpath.
• bash: write wtk completion to a user-writable bash completion directory when available.
• fish: write wtk.fish to ~/.config/fish/completions/.
• PowerShell: document or support profile integration if feasible.
• Keep printing manual setup instructions when automatic installation is unsupported or unsafe.
• Provide an opt-out environment variable, e.g. WTK_SKIP_COMPLETION_INSTALL=1, for users who do not want installer-side shell configuration.

Acceptance criteria

• scripts/install.sh attempts completion installation only after the binary is installed and verified.
• The installer does not require elevated privileges for completion setup.
• Unsupported shells or non-writable completion directories produce clear guidance instead of pretending setup succeeded.
• Existing manual wtk completion <shell> behavior remains available.
• Installer tests cover successful auto-install, opt-out, and unsupported/unsafe cases.

Notes

This should make the one-click install feel complete: after installation, users can immediately use tab completion in a supported shell without extra setup.

Start with scripts/install.sh and the existing wtk completion <shell> behavior to understand the installation flow and supported shells. Define safe user-writable completion locations and the opt-out path before adding installer tests. Done means verified installs can configure supported shells without elevated privileges, while unsupported or unsafe cases retain clear manual guidance.