install.sh: offer to configure PATH and shell completions in shellrc

[murrayju]

Summary

The install script now helps users finish setup by offering (with a y/N confirmation prompt) to:

1. Add the install directory to PATH in the user's shellrc when it isn't already there.
2. Enable shell completions by appending a source <(ghost completion <shell>) snippet to the shellrc when completions aren't already configured.

Both prompts work under curl | sh (they read from /dev/tty). Declining a prompt or running non-interactively falls back to printing the exact manual command the user would need to run, so the installer stays useful in CI / pipelines.

Modeled on the equivalent logic in ox's install.sh, adapted to stay POSIX/dash-compatible (the Ghost installer's existing shebang is #!/bin/sh).

Details

Shellrc detection (detect_shell_rc) uses $SHELL:

• zsh → ${ZDOTDIR:-$HOME}/.zshrc
• bash → $HOME/.bash_profile on macOS when it exists, otherwise $HOME/.bashrc
• fish → ${XDG_CONFIG_HOME:-$HOME/.config}/fish/config.fish
• Unknown → .zshrc on Darwin or when it already exists, otherwise .bashrc

PATH configuration (configure_path_in_shellrc):

• No-op when the dir is already in PATH or already referenced in the shellrc.
• Uses fish_add_path / set -gx PATH for fish, export PATH="$dir:$PATH" otherwise.

Completion configuration (configure_shell_completions):

• Supports bash, zsh, fish. Silently skips for unknown shells with a pointer to ghost completion --help.
• For zsh, detects whether compinit is already loaded (directly or via oh-my-zsh, prezto, zinit, antigen, zplug, or zgenom) and offers to add autoload -Uz compinit && compinit -i when it isn't.
• Uses ghost completion fish | source for fish, source <(ghost completion <shell>) otherwise.

Removed the static PATH warning that lived inside verify_installation — that responsibility now belongs to configure_path_in_shellrc, which can actually fix the problem instead of just warning about it.

Testing

• sh -n scripts/install.sh passes.
• Manually tested the helper functions in isolation for zsh / bash / fish / unknown $SHELL values; each returns the expected rc path, shell type, and completion snippet.