tsk-49: prop list prints keys only
`tsk prop list -T tsk-N` now prints one key per line instead of
expanding each value as a separate (key, value) row. Values are
still reachable via `tsk show -x` (YAML front-matter) or
`tsk prop find <key>`. AGENTS.md updated to match.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>