fix(compliance): remove stale vector count from signed-requests-runner header

[bokelley]

Closes #6071

The header comment in static/compliance/source/test-kits/signed-requests-runner.yaml said "28 conformance vectors" but the actual set has been 40 (12 positive + 28 negative) since 3.1.0. The count was never accurate for any released version — at 3.0.x the set was 39 (12 + 27), and "28" only happened to equal len(negative/) after vector 028-unsigned-protocol-method-required was added in 3.1.0. This PR removes the hard-coded count and replaces it with a directory reference (positive/ and negative/), matching the style used by the storyboard's own pass_criteria block, which deliberately avoids stating a total for the same reason.

Non-breaking justification: Comment-only change in a YAML source file. No runner logic, schema field, or generated output is affected. Released dist artifacts (dist/compliance/3.1.4/) remain immutable and carry the old wording by design; the fix ships into the next patch via source.

Pre-PR review:

• code-reviewer: approved — no blockers; one wording nit addressed (article "the" before bare dir name → "vectors in positive/ and negative/ at …")
• ad-tech-protocol-expert: approved — wording correct, patch bump level correct per patch-eligibility table (non-normative compliance harness comment); dist immutability rule confirmed unaffected

Triage-managed PR. This bot does not currently iterate on
review comments or PR conversation threads (only on the source
issue). To unblock:

• Push fixup commits directly: gh pr checkout <num> →
  fix → push.
• Or request a new first draft PR: comment /triage execute
  on the source issue only when no triage-managed PR is already
  open. Triage does not update existing PRs.

See #3121
for context.

Session: https://claude.ai/code/session_01EYYPbYTWZUnMa1KXb8tJZT

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