Keep one extras list per device instead of per version

The package lists were stored as config_<version>/packages_<device>_<version>.txt,
but the version dimension was pure duplication: the Linksys MX8500 extras were
byte-identical across 24.10.4, 25.12.0, 25.12.2 and 25.12.4, and the Cudy lists
only ever changed with the device, never with the release.

Your packages now live in packages/<device-id>.txt, one file per device and
independent of the OpenWrt version. The built-in part is fetched per release
anyway, so upgrading needs no file changes at all.

The combined list is a build artefact and moves to .generated/, which is
gitignored, along with the buildinfo saved by --save-buildinfo --offline.

This removes work rather than adding it: because the generated file is now
entirely machine-owned, the sentinel comments, the in-section replacement state
machine and the "line containing base-files and libc is the old generated one"
migration heuristic are all gone. Hand-written content is isolated in the extras
file, generated content in .generated/.

Also fix version detection when a script is run from the helper directory. The
helper checkout is its own git repository inside the OpenWrt worktree, so git
commands there described the helper repo -- a checkout of OpenWrt 25.12.5 was
reported as SNAPSHOT because the helper repo is on main. Version queries are now
anchored to the OpenWrt tree.

The config_* archives are deleted; git history keeps them. That includes
config_24.10.4/apply-dahdi-patches.sh, which only applied to 24.10.4.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,3 @@
+# Generated artefacts: full package lists and saved buildinfo.
+# Regenerate with gen-package-list.sh / download-config.sh --save-buildinfo
+.generated/