fix(state): resolve home-based paths from injected env for cross-platform parity

os.homedir() ignores $HOME on macOS, so the XDG-migration test's HOME
override did not isolate the legacy ~/.opencode-goal-plugin fallback and
collided with a real user state file. Resolve the home base from an
injectable env.HOME (falling back to os.homedir()), thread env/cwd through
GoalPlugin, and make the migration test inject config instead of mutating
process globals. No production behavior change where env.HOME == real home.