Isolate lease reading in a subagent

Give the subagent a narrow job and a fixed reply format, because its reply is all the main session ever sees. The decisive part of lease-reader.md isn't the tool list; it's the reply contract: at most 300 words, exact quotes with clause numbers, and a separate line for conditions and exceptions, since those are what summaries drop first. The harness asks the three questions headlessly, confirms the main session delegated, and checks each reply for the phrases that must survive.