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

examples/git-hooks/pre-push

@@ -28,6 +28,12 @@ if [ ! -d .beads ]; then
     exit 0
 fi
 
+# Skip if bd sync is already in progress (GH#532: prevents circular error)
+# bd sync sets this env var when pushing from worktree
+if [ -n "$BD_SYNC_IN_PROGRESS" ]; then
+    exit 0
+fi
+
 # Check if sync-branch is configured in config.yaml or env var
 # If so, .beads changes go to a separate branch via worktree, not the current branch
 SYNC_BRANCH="${BEADS_SYNC_BRANCH:-}"