cache: git-tracked build cache — 16 packages, survives make clean + clone

Red Bear is a fork/overlay on top of Redox. The upstream build
system wasn't designed for forks — it loses all cached stages on
make clean with no recovery path.

This commit adds a git-tracked build cache:
- local/cache/pkgar/{pkg}/stage.pkgar — per-package cache files
- cache-sync.sh: sync built packages → git-tracked cache
- cache-sync.sh --restore: restore cache → recipe targets
- cache-sync.sh --commit: sync + git commit
- Auto-restore before build, auto-sync after build

Cache survives: make clean, make distclean, git clone, upstream rebase.
Recovery from clean: seconds (restore from git) vs hours (full rebuild).

local/cache/pkgar/.gitkeep

@@ -0,0 +1 @@
+# Red Bear package caches — committed to git