Testflinger CLI Multi-Value Argument Inconsistencies

Some of these "issues" are in the works, and so aren't real problems yet. However, that for example the `--key` argument; the example help string indicates that the expected syntax will be what has been fixed to match in the `curses` branch. (Note: curses branch does not use curses, it was poorly named)

Testflinger CLI has **3 different ways** to pass multiple values.

Summary of all variants found (Style (branch): example):
- Repeated flag referenced in help text for `reserve` subcommand: `--key lp:user --key gh:user`
- action="append"
- singular
- Space-separated (`main`): `--agents agent1 agent2`
- action="extend" + nargs="+"
- plural
- Comma-separated (`status-list`): `--fields name,status,location`
- No action= or nargs= specified
- plural
- `default="name,status,location,provision_type,comment"`
- Single string value
- Code manually splits on comma in processing: `fields = [f.strip() for f in self.args.fields.split(",")]`

Start by locating the CLI argument definitions for the reserve, main, and status-list subcommands, then compare them with the behavior described for the curses branch. A maintainer decision is needed on one multi-value convention before changing help text, parsing, and processing. Done means the affected arguments use the agreed convention consistently and their examples match it.

Written by the indexing model from the issue text.