clusterdiscovery: recompute account per op; cache only cluster->cores

Reworks ResolveContextForCluster around the cluster_cores cache and a fresh per-operation account selection, replacing the cluster->context binding model:

• Cluster cores come from cluster_cores.json (TTL'd, stale-fallback), not a persisted account choice.
• Account selection: active context wins if its core fronts the cluster; else the sole eligible context; else (>1 eligible, none active) a hard error listing the candidates and pointing at entire auth use. No more silent on-disk-order pick, which could authenticate as the wrong account and then freeze that choice.

Drops the post-success bind hook from git-remote-entire (makeBindHook) and adds a cacheDir argument to ResolveContextForCluster. The binding APIs in contexts/ and the auth command surface are removed in a follow-up.