Running the installer or `codegraph init`/`index` from $HOME auto-indexed the
entire home tree (installer indexes process.cwd() with no guard), producing a
multi-GB ~/.codegraph/codegraph.db; the install dir sharing the ~/.codegraph
name then made every home subdir resolve its root to $HOME. On pre-1.0 macOS the
per-file watcher over that tree exhausted kern.maxfiles and crashed the machine
(#845; the fd blowup was fixed in 1.0.0, this fixes the root cause).
Add unsafeIndexRootReason() and refuse the home dir, a parent of home, and
filesystem roots at the installer auto-index, `init`, and `index`. Overridable
with --force. Closes#845.
