journal showed three concurrent rebuilds racing on the meta
repo's .git/index.lock — auto_update::run kicks off parallel
tokio::spawn for every stale agent, each rebuild eventually
calls into meta::sync_agents / lock_update_for_rebuild which
do git add + commit, git isn't safe across concurrent processes
on the same .git/, and one of the failing-mid-write children
left index.lock behind. subsequent ops blocked until somebody
rm'd it manually.
fix: static META_LOCK (tokio::sync::Mutex<()>) acquired at the
top of every public meta function. concurrent rebuilds take
turns on meta ops; the actual nix build (nixos-container update)
releases the lock first and runs without it, so parallel agent
builds still parallelize on nix-daemon's own concurrency model.
migrate::run additionally clears /var/lib/hyperhive/meta/.git/
index.lock on startup if it exists — we just booted, nothing
of ours is holding it. covers the 'previous crash left a stale
lock' case the user just hit so the daemon recovers without
manual intervention.
