FEATURE: h8 app describe --field for filtered output

## 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.

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

## Proposal

```bash
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

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