fix: print PATH setup instructions when install dir is not in PATH

[kylecarbs]

On macOS, /usr/local/bin is not user-writable (Homebrew lives in /opt/homebrew on Apple Silicon), so the installer falls back to ~/.local/bin, which is not in the default macOS PATH. The script only printed a bare warning that was easy to miss in curl | sh output and offered no remediation.

The install location logic is unchanged. When the install directory is not in PATH, the script now:

• Prints the exact line to append for the user's login shell (~/.zshrc, ~/.bashrc, or fish_add_path), using $HOME-relative paths where possible.
• Mentions rerunning with sudo BOO_INSTALL_DIR=/usr/local/bin for a system-wide install.

Example output for a zsh user:

warning: /Users/me/.local/bin is not in your PATH
To add it, run:
  echo 'export PATH="$HOME/.local/bin:$PATH"' >> ~/.zshrc
then restart your shell.

For a system-wide install instead, rerun with:
  curl -fsSL https://raw.githubusercontent.com/coder/boo/main/install.sh | sudo BOO_INSTALL_DIR=/usr/local/bin sh

Testing and decisions

• Verified with shellcheck (clean).
• Ran the script end-to-end with SHELL set to zsh, bash, fish, and dash; each prints the correct instructions, and no warning is printed when the install dir is already in PATH.
• Chose actionable messaging over sudo escalation: prompting for sudo inside curl | sh hangs unattended installs, and modern installers (rustup, uv, bun) install user-local without sudo.

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Generated by Coder Agents on behalf of @kylecarbs.