fix(sync): prevent circular error in pre-push hook (GH#532)

When bd sync pushes from the sync-branch worktree, the pre-push hook
would detect uncommitted JSONL changes and suggest running bd sync -
which is circular since that's what the user is already doing.

Fix: Set BD_SYNC_IN_PROGRESS=1 environment variable when pushing from
worktree, and update pre-push hook to skip checks when this var is set.

Updated files:
- internal/syncbranch/worktree.go: Set env var on push command
- cmd/bd/templates/hooks/pre-push: Check for env var and exit early
- examples/git-hooks/pre-push: Same check
- .beads-hooks/pre-push: Same check

.beads/.gitignore

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