install script ignores OPENCODE_INSTALL_DIR and XDG_BIN_DIR documented in README

Description

The install script (install in the repo root) hardcodes INSTALL_DIR=$HOME/.opencode/bin and never reads $OPENCODE_INSTALL_DIR or $XDG_BIN_DIR, but the README documents a priority order for the installation directory:

1. $OPENCODE_INSTALL_DIR - Custom installation directory
2. $XDG_BIN_DIR - XDG Base Directory Specification compliant path
3. $HOME/bin
4. $HOME/.opencode/bin (default)

and even shows usage examples:

OPENCODE_INSTALL_DIR=/usr/local/bin curl -fsSL https://opencode.ai/install | bash
XDG_BIN_DIR=$HOME/.local/bin curl -fsSL https://opencode.ai/install | bash

So the documented examples do not work: the binary is always installed to $HOME/.opencode/bin regardless of the variables.

Steps to reproduce

1. Run OPENCODE_INSTALL_DIR=/tmp/foo curl -fsSL https://opencode.ai/install | bash (or run the repo's install script with the variable set)
2. Observe the binary is installed to $HOME/.opencode/bin, not /tmp/foo
3. Same with XDG_BIN_DIR=$HOME/.local/bin

Plugins

N/A

OpenCode version

Latest install script on dev branch