#readable returns false for an agent session, so a `*` share no longer shows a user's message in an agent's trail. New tests: a sharee's session.list refuses (Q1), and an agent bound to the shared thread, a chief run and the tester match no share (Q2). BUILD.md records the Q1 follow-up, Q2-Q4 and the live --migrate as Mos's go under Q173. README names the agent rule.