.beads/interactions.jsonl is beads' agent audit trail sidecar — bd writes
one line per mutation (claim, assign, comment, status change). The July bd
release's `bd init` created it as a git-tracked file and omitted it from
.beads/.gitignore by design, so every fragua run that claims or comments
dirtied the worktree with an 'M .beads/interactions.jsonl'.
Newer beads made the sidecar opt-in (audit.enabled default false) and
gitignore it. This commit backports that convention:
1. Adds 'interactions.jsonl' to .beads/.gitignore
2. git rm --cached to stop tracking (file kept on disk)
