feat: add `humans.txt` presence as a scoring check

[vinpatel]

A /humans.txt file signals the site is maintained by real humans (team, contact, stack). It's a low-weight but well-known trust signal — a reasonable fit for the Trust & Authority pillar.

Scope

Add a new scoring check that:

• Fetches /humans.txt alongside the other root-level files
• Returns a binary pass/fail (or a small partial-credit tier if the file is non-empty and structured)
• Weights low (1-2% of the pillar) so it's a nice-to-have, not a score driver

Where to add it

Scoring logic lives in packages/core/src/scorer/. The pillars/dimensions are defined in packages/core/src/scorer/dimensions.ts. Look at how an existing small check like `ai.txt` is wired — humans.txt will mirror its shape.

Acceptance

• Check implemented, unit tested, and wired into the Trust & Authority pillar
• Documented in `apps/docs` under the scoring criteria reference
• Existing tests still pass (`pnpm -r test`)

Good first issue because it's a well-scoped single-file change with clear reference patterns. ~1-2 hours.