Implements ifcurl-5oo. get_checkout() ensures the bare cache is current via
fetch_ifc(), then clones it locally to <cache_dir>/checkout/ (no network),
and places it at the resolved commit as a detached HEAD — forcing branch
creation before any commit. Viewer artefact patterns (*.ifcview, *.rdbview,
etc.) are appended to .git/info/exclude so they are locally ignored without
touching the repo's own .gitignore. When a newer commit is requested, the
checkout fetches from origin (the local bare cache) using the default refspec
to avoid git's "refusing to fetch into checked-out branch" error. Unblocks
the JSON-RPC connector (ifcurl-242).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
