ci: build the PC and Quest clients on Jenkins

A Jenkinsfile builds, on push (only what changed) or on demand:
- the PC patcher zip (MelonLoader, the mod, config template, Steam stub),
- the full PC client (stock build + patcher + config),
- the Quest APK and .xapk.

Stock builds come out of the restic build archive on the FTP: restic
through its rclone backend, picked by snapshot tag per run and cached.
The FTP login and restic password are Jenkins credentials, and the
repository folders are environment settings, so nothing about the
archive is fixed in the repo.

ci/jenkins/Dockerfile is the Jenkins image with the toolchain added
(.NET SDK, Android NDK and build-tools, apktool, Python with LIEF,
restic, rclone), for the existing Jenkins service.

The build scripts move from PowerShell to Python so they run the same
on Windows, Linux and in CI:
- DorkNet.Quest/scripts/build.py replaces build.ps1 + package.ps1; it
  can use a committed hook table (checked against the APK's
  libil2cpp.so) instead of dump.cs, and takes a persistent keystore.
- tools/build_pc.py replaces build-patcher-zip.ps1 (now with the
  2023 install layout) and adds the full-client build.

Setup steps are in ci/jenkins/README.md.

ci/jenkins/.gitattributes

@@ -0,0 +1,2 @@
+# Shell scripts run inside the Linux image; keep them LF on every checkout.
+*.sh text eol=lf