ci: keep the checkout so the path filter can skip non-client pushes

The job's path filter needs the last build's checkout to compare against.
The pipeline deleted the whole workspace after each run, so Jenkins polled
with "Working Directory does not exist", assumed changes, and started a
run for every push (builds #6 and #7: zero changed files). Delete only
out/ and quest-work/ instead.

ci/jenkins/README.md

@@ -71,7 +71,10 @@ Resolve a table for another build locally with `build.py build --dump-cs`.
   under the job's **Git** source, add **Additional Behaviours → Polling
   ignores commits in certain paths** with included regions
   `DorkNet[.]ClientMod/.*`, `DorkNet[.]Quest/.*`, `tools/build_pc[.]py` (one
-  per line) and excluded region `.*[.]md`.
+  per line) and excluded region `.*[.]md`. The filter compares against the
+  last build's checkout, so the pipeline keeps it and deletes only `out/`
+  and its scratch folders after each run; without a checkout, Jenkins treats
+  every push as a change.
 - **Build with Parameters** runs a manual build:
   - `PLATFORMS` (both / pc / quest), and `PC_FULL_CLIENT` for the full PC client;
   - snapshot tags, and per-run repository overrides;