feat(cli): install shell completion loaders

[rbalachandar]

Follow-up to #89, which adds the aenv completion command and visible aliases. This PR is intentionally scoped to automatic installation and removal.

What it does

• Installs lightweight Bash, Zsh, and Fish completion loaders into standard completion directories.
• Uses the installed aenv binary path so loaders do not depend on an unrelated PATH binary.
• Canonicalizes prefix and HOME paths before selecting user versus system mode.
• Never modifies .bashrc, .zshrc, or Fish configuration files.
• Leaves unmanaged completion files and symlink destinations untouched.
• Atomically stages managed completion files in their destination directory and supports managed upgrades.
• make uninstall-aenv removes only files owned by the completion installer.
• Prints actionable Zsh fpath and compinit guidance for user-local installs.
• Treats staging and filesystem failures as non-fatal warnings so a CLI install is not aborted by completion setup.

The standalone installers remain self-contained because they are distributed as curl | bash entrypoints; their small loader-writing snippets mirror the shared Make/test helper without the previous shell configuration manager or synchronization machinery.

Validation

• bash -n for all installer and test scripts
• ShellCheck for all installer and test scripts
• bash scripts/tests/verify-completion-install.sh
• make check-completion-install
• Functional coverage includes loader syntax, 0644 modes, install, managed upgrade, unmanaged Bash/Zsh/Fish preservation, symlink preservation, path traversal classification, staging-failure cleanup, standard system paths, and uninstall.

This draft should be reviewed after #89 is merged, since the loaders invoke the completion command introduced there.