Agreements Library: 38 standard agreements, word for word, with their sources

Internal: the Agreements Library (site-admin preview) now holds 38 standard agreements: Y Combinator's ten, plus NDAs, professional services, cloud service, software licence, pilot, design partner, partnership, data processing and business associate agreements and more from Common Paper, Bonterms, oneNDA, Papertrail and Series Seed. Each is built by tools/agreement-forms/ from its publisher's original file, kept in the repository with its download address, time and SHA-256. The only changes are logged edits: blanks, the form's own alternatives and tick boxes, signature and date lines, and notes the publisher says to remove. AgreementSourceFidelityTest checks every word against the original on each test run, and docs/AGREEMENT_SOURCES.md records each source, its licence quoted, and every change. Agreements can now ask yes-or-no and number questions, ask a question only for some answers, leave out rows the form says to delete, be signed in more than one place with Date signed fields, and be texts no one signs, which are downloaded instead of sent. The Library is grouped by what an agreement is for. None of the texts has been reviewed by a lawyer.