✨ Add daily company change summaries

[AnandChowdhary]

Summary

• Adds a reusable company diff builder that compares the previous and freshly fetched YC company records by stable company ID.
• Publishes dated and latest change summaries in both JSON and Markdown under changes/ whenever company records change.
• Updates generated metadata/README when any company record changes, not only when aggregate counts change.
• Seeds today’s latest change summary from the live YC fetch: 2 added companies and 5 updated companies.

Test Plan

• deno task check
• deno task update — wrote changes/2026-06-30.{json,md} and changes/latest.{json,md}
• Re-ran deno task update — confirmed idempotency with "No company change files written" and "API data has not changed"