FEATURE: h8 app describe --field for filtered output

[erfanium]

Problem

h8 app describe grist --json returns 100+ line JSON with nested org, user, cluster, and workflow objects — 95% of which is irrelevant when you just need the envs, disk, or plan.

h8 app describe grist --json | wc -l   # ~120 lines

Proposal

h8 app describe grist --field envs,disk       # only envs and disk
h8 app describe grist --field command         # check current entrypoint
h8 app describe grist --field plan,replicas   # sizing info

Alternative

h8 app describe grist | grep envs             # works but ugly
python3 -c "import sys,json; ..."             # too much typing