`miren sandbox-pool list` takes ~10s on garden and has no filters

miren sandbox-pool list -C garden takes about 10 seconds to return 40 pools. Three runs today took 11.3s, 10.9s and 9.9s, and the JSON output is just as slow, so it isn't the table rendering. miren app list against the same cluster returns in 0.3s, which points at the pool listing path rather than the connection. Garden is on server main:045fc2f and the CLI is v0.17.2.

Pool listing moved behind the SandboxPools RPC in MIR-2087, and the authorization work after it added per-pool app visibility filtering. Both are worth checking first for a per-pool round trip or lookup.

The command also can't narrow its output. A cluster with dozens of pools needs at least an --app filter, and probably --service. Today the only way to find an app's pools is to read the whole table. Several rows show no version or service at all (addon pools, for example), so it's hard to tell what's what. MIR-1361 covers the missing app name column, and we should land both together.

Done when: listing garden's pools returns in well under a second, --app filters to one app's pools, and the table shows each pool's app (MIR-1361).