airfield: stop tracking .air (per-machine local config)

The .air file is airfield's per-machine local config: it lists container mounts, including host paths that embed the login user's numeric uid (/run/user/<uid>/gdm). This repo is shared course infrastructure -- cloned by students on lab machines and adapted for other robot platforms -- so host-specific values do not belong in it.

The consuming project now keeps those mounts in its own project-level .air, which is gitignored there and also survives a re-clone of this repo by checkout.sh.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: JacobTaira

.air

@@ -13,3 +13,8 @@ config/car.lua.*
 .airfield/
 # core dumps (e.g. a crashed node leaves /core in the workdir)
 /core
+# airfield per-machine local config (container mounts). This repo is shared
+# course infrastructure -- students on lab machines and other robot platforms
+# clone it -- so host-specific paths (uids, mount points) must not be committed
+# here. Keep them in the consuming project's own .air instead.
+.air