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

internal/syncbranch/worktree.go

@@ -728,6 +728,9 @@ func pushFromWorktree(ctx context.Context, worktreePath, branch string) error {
 	for attempt := 0; attempt < maxRetries; attempt++ {
 		// Push with explicit remote and branch, set upstream if not set
 		cmd := exec.CommandContext(ctx, "git", "-C", worktreePath, "push", "--set-upstream", remote, branch)
+		// Set BD_SYNC_IN_PROGRESS so pre-push hook knows to skip checks (GH#532)
+		// This prevents circular error where hook suggests running bd sync
+		cmd.Env = append(os.Environ(), "BD_SYNC_IN_PROGRESS=1")
 		output, err := cmd.CombinedOutput()
 
 		if err == nil {