fix(search): infer support query domain from SUPPORT_TAGS

[hobostay]

Summary

inferQueryDomain (in src/utils/search-index.ts) only scores TECHNICAL_TAGS and OPS_TAGS — it never looks at SUPPORT_TAGS. As a result any query made of support tokens (faq, onboarding, guide, user, …) is classified as neutral.

This is both a ranking bug and an internal inconsistency:

• Under DOMAIN_WEIGHT.neutral, support entries get weight 0.3 while neutral entries get 0.85. So a genuine support question ranks a matching support document below an unrelated neutral document — the opposite of the intended ranking.
• It is asymmetric with inferDomain (lines ~167–183), which scores all three tag sets, and contradicts inferQueryDomain's own doc comment: "Uses the same tag sets used for document domain inference so the two sides of the matching are symmetric."

Fix

Score SUPPORT_TAGS as well, mirroring inferDomain:

function inferQueryDomain(queryTokens: string[]): KnowledgeDomain {
  let techScore = 0, opsScore = 0, supportScore = 0;
  for (const t of queryTokens) {
    if (TECHNICAL_TAGS.has(t)) techScore++;
    if (OPS_TAGS.has(t)) opsScore++;
    if (SUPPORT_TAGS.has(t)) supportScore++;
  }
  const maxScore = Math.max(techScore, opsScore, supportScore);
  if (maxScore === 0) return 'neutral';
  if (techScore === maxScore) return 'technical';   // tie-break: technical > ops > support
  if (opsScore === maxScore) return 'ops';
  return 'support';
}

Test

Added a regression test (search-domain-weighting.test.ts) that builds a real index with two identical-score entries — one domain: support, one domain: neutral — and queries with support tokens. It asserts the support entry outranks the neutral one.

• Before fix: fails — support entry scores 2.4, neutral 6.8 (support penalized ×0.3 under the mis-inferred neutral query domain).
• After fix: passes — query domain is support, support entry outranks neutral.

npm test → 1440 passed; tsc --noEmit → clean.