Fix shell completion installation instructions for Linux

[robzolkos]

The current completion instructions fail on Linux for bash and zsh because they write to system directories without proper root access.

Problem

Shell redirection (>) is executed by the current shell, not by sudo, so these fail with Permission denied:

Bash
$ bcq completion bash > /etc/bash_completion.d/bcq <-- fails

Zsh (when fpath[1] is a system directory)
$ bcq completion zsh > "${fpath[1]}/_bcq"

Proposed fix

Follow GitHub CLI's approach - recommend user-local methods as primary:

Bash:
To load completions for every new session, add to ~/.bashrc:
eval "$(bcq completion bash)"

Or install system-wide (requires root):
$ bcq completion bash | sudo tee /etc/bash_completion.d/bcq > /dev/null

Zsh:
To load completions for every new session, add to ~/.zshrc:
eval "$(bcq completion zsh)"

Or install to a user directory in your fpath:

  $ mkdir -p ~/.zsh/completions
  $ bcq completion zsh > ~/.zsh/completions/_bcq
  # Then add to ~/.zshrc: fpath=(~/.zsh/completions $fpath)