Add HTTP Archive report: SEO / llms.txt adoption

[max-ostapenko]

Query:

-- valid = TRUE means "present" or "exists"
WITH adoption AS (
  SELECT
    date,
    client,
SAFE.BOOL(custom_metrics.other.llms_txt_validation.valid) AS adoption
  FROM
    `httparchive.crawl.pages`
  WHERE
    date = '2025-07-01' AND
    is_root_page = TRUE
)

SELECT
  date,
  client,
  SAFE_DIVIDE(COUNTIF(adoption), COUNT(0)) AS pct_pages
FROM adoption
GROUP BY
  date,
  client;