Open house visitors say whether they already work with an agent, and the agent sees it on the visitor list and the CSV #138

For: the agent knows which open house visitors are unrepresented buyers worth a follow-up call, as the printed sheet already asks.

Done looks like:

• [ ] The tablet sign-in page (/listings/:id/sign-in) asks "Are you working with an agent?" with Yes / No (optional), validated at the boundary; anything else is refused.
• [ ] The listing's visitor list shows the answer for each visitor, and /listings/:id/visitors.csv has a column for it.
• [ ] Existing visitors without an answer read as blank, with a migration that keeps old data.
• [ ] The thank-you screen still never shows earlier visitors.
• [ ] The sample listing's visitors include both answers.
• [ ] bun test passes; a test fails without the new field on the form, the list and the CSV.

Files likely: src/pages/openhouse.ts or the sign-in page module, src/db.ts (column + migration), src/actions.ts, src/sample.ts, src/signin.test.ts.

Risk: the visitor table migration on production data; add the column, do not rebuild the table.