fix(install): create ~/.zshrc when it's missing so opencode lands on PATH

[kitlangton]

Today

A new Mac has no ~/.zshrc. When someone runs curl -fsSL https://opencode.ai/v2/install | bash on one, the installer finds no zsh config file, prints No config file found for zsh. You may need to manually add to PATH, and stops there. The next command they type, opencode, fails with command not found.

After

When none of the zsh (or fish) config files exist, the installer creates the shell's main one (${ZDOTDIR:-$HOME}/.zshrc, or ~/.config/fish/config.fish) and adds the PATH line to it as usual. Bash and other shells keep today's warning, because on macOS a login bash reads .bash_profile and guessing the wrong file would hide the problem.

How

install: before the "No config file found" branch, pick the shell's main config file, mkdir -p its folder and touch it. If either step fails, the old warning still prints.

Proof

On a throwaway HOME with SHELL=/bin/zsh and no dotfiles:

$ env -i HOME=/tmp/onb-home SHELL=/bin/zsh PATH=/usr/bin:/bin bash install
$ cat /tmp/onb-home/.zshrc

# opencode
export PATH=/tmp/onb-home/.opencode/bin:$PATH

A second run prints Command already exists in /tmp/onb-home/.zshrc, skipping write., and --no-modify-path still creates no files.

Risk

Low. It only runs when no candidate file exists, and only for zsh and fish. Undo is a one-line revert.