refactor(install): split dev env install from service install
make install is now repo-local dev env only and no longer writes to
user-level aliases or skills. make install-service becomes the single
smart install-or-upgrade path that owns ~/.local/bin/sol, the global
solstone skill, and the background service.
add think.install_guard to enforce alias ownership before any
user-level mutation. cross-repo, dangling, not-a-symlink, and worktree
states hard-fail without a bypass. remove the upgrade target, make
uninstall an ambiguity error, and align the docs with the new install
split.