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.

microsoft/knowledge/security/use-secrettext-for-credentials.md

@@ -9,8 +9,6 @@ application-area: [all]
 
 # Use SecretText for credentials
 
-> **Seed article.** Converted from an existing security-review prompt to bootstrap the BCQuality security corpus. Domain stewards should expand, restructure, and refine as needed.
-
 ## Description
 
 SecretText is a compile-time-checked AL type for credentials, API keys, tokens, and similar sensitive values. The compiler rejects literal assignments to SecretText and blocks implicit conversion back to Text or Code, which prevents many accidental disclosures via logs, errors, and the debugger (regular and snapshot). A SecretText value remains opaque throughout its lifetime.