gitignore .beads/interactions.jsonl (untrack beads audit sidecar)

.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)

.beads/.gitignore

@@ -71,6 +71,10 @@ backup/
 *.db-shm
 db.sqlite
 bd.db
+
+# Interactions log (runtime, not versioned)
+interactions.jsonl
+
 # NOTE: Do NOT add negation patterns here.
 # They would override fork protection in .git/info/exclude.
 # Config files (metadata.json, config.yaml) are tracked by git by default