Branching: render merge/sync conflicts as a readable table on a TTY #169

On a terminal, a conflict envelope (exit 10) prints details.conflicts as a raw Python dict via render_to_rich_stderr (nsc/output/errors.py:101). The design asks for a rendered list: object, conflicting fields, and the original / branch / main values.

Fix: when the envelope has details.conflicts and the target is rich stderr, render a table with one row per conflict. JSON output stays unchanged.

Deferred minor from the final review of #167 (NetBox Branching, #166).