chore: remove legacy deletions.jsonl after tombstone migration

Tombstones are now stored inline in issues.jsonl.
Added deletions.jsonl to .gitignore to prevent re-tracking.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.beads/.gitignore

@@ -30,3 +30,5 @@ beads.right.meta.json
 !issues.jsonl
 !metadata.json
 !config.json
+deletions.jsonl
+deletions.jsonl.migrated