Triage seed knowledge and document admission test for preview

Remove seven knowledge files whose content is generic software-engineering
guidance that a capable LLM already applies without BCQuality present
(HTTPS-only, secret-leakage-in-errors, no-credentials-in-URLs, silent
security-error swallowing, short transaction scope, HTTP timeouts,
StrSubstNo-vs-concatenation). These fail the remedial-knowledge premise
and dilute the signal of the preview corpus.

Strip the "Seed article — domain stewards should expand" banner from ten
files that are ready to showcase (AA0232/AA0233 rules, FindSet read-only
semantics, SetLoadFields ordering and usage, CalcFields-in-loops,
SecretText end-to-end, DataClassification). The banner remains on files
that still need domain-steward refinement.

Add a "What belongs here" section to the README stating the admission
test: a file exists only if a modern LLM would get something wrong or
miss something without it. Gives contributors a concrete yes/no filter
before they open a PR.

community/knowledge/performance/call-setloadfields-before-filters.md

@@ -9,8 +9,6 @@ application-area: [all]
 
 # Call SetLoadFields before filters
 
-> **Seed article.** Ported from BC Code Intelligence to seed the community corpus. Community contributors are invited to expand or refine.
-
 ## Description
 
 `SetLoadFields` is folded into the database query that the subsequent `Find`, `FindSet`, or `FindFirst` executes. When it is called after filters have already been applied, the platform either ignores the specification or is forced into an extra round-trip to reload the narrower column set — negating the optimization. The placement rule is simple and absolute: `SetLoadFields` must come first.