Run the custody session through a Make target, and give the runner a shebang

The runner was executable with no shebang, so a shell handed its path tried to
interpret Python as shell script. That is what a broken command line hit, and it
would have hit anyone invoking the file directly.

Also adds `make openbao-open-questions`, which is short enough that terminal
wrapping cannot split it, refuses a receipt path that is missing or already
exists, and clears OPENBAO_TOKEN/VAULT_TOKEN/BAO_TOKEN before invoking the
attended envelope so the identity comes from the login rather than a standing
token.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WLUjpv3ssxNRAEPPgLFnEB

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 1275505@bnt-lap001
Assistant-Session: 97265baa-f08f-4032-b290-a1e2965a69c5

scripts/openbao_open_questions_session.py

@@ -1,3 +1,4 @@
+#!/usr/bin/env python3
 """Read-only, silent owner session for the four open custody questions.
 
 Runs inside Warden's attended login envelope. Reads no secret value into the