fix(#2198): remove dead scan exports, correct injection-scan docs

scanEntropyAnomalies + shannonEntropy were dead exports with zero
production callers — the live hooks (gsd-prompt-guard.js,
gsd-read-injection-scanner.js) inline their own pattern subsets for
hook independence and never called these functions.

Changes:
- Remove scanEntropyAnomalies + shannonEntropy from src/security.cts
- Remove scanEntropyAnomalies test block from tests/security.test.cjs
- Correct REQ-SCAN-INJ-02/-03 in FEATURES.md (EN/zh-CN/ja-JP) to
  describe what actually runs live (injection patterns, invisible
  Unicode) vs CI-only (base64-decode, codebase scan)
- Correct docs/security/baseline.md §2.4 to clarify live hooks inline
  patterns, not import from security.cts
- Add regression test asserting the corrected contract
- scanForInjection retained: it serves as the CI codebase-scanner engine

src/security.cts

@@ -477,40 +477,9 @@ export function validatePromptStructure(text: unknown, fileType: string): { vali
   return { valid: violations.length === 0, violations };
 }
 
-// ─── Layer 4: Paragraph-Level Entropy Anomaly Detection ─────────────────────────────────────────────────────────────────────
-
-function shannonEntropy(text: string): number {
-  if (!text || text.length === 0) return 0;
-  const freq: Record<string, number> = {};
-  for (const ch of text) {
-    freq[ch] = (freq[ch] || 0) + 1;
-  }
-  const len = text.length;
-  let entropy = 0;
-  for (const count of Object.values(freq)) {
-    const p = count / len;
-    entropy -= p * Math.log2(p);
-  }
-  return entropy;
-}
-
-/**
- * Scan text for paragraphs with anomalously high Shannon entropy.
- */
-export function scanEntropyAnomalies(text: unknown): { clean: boolean; findings: string[] } {
-  if (!text || typeof text !== 'string') {
-    return { clean: true, findings: [] };
-  }
-  const findings: string[] = [];
-  const paragraphs = text.split(/\n\n+/);
-  for (const para of paragraphs) {
-    if (para.length <= 50) continue;
-    const entropy = shannonEntropy(para);
-    if (entropy > 5.5) {
-      findings.push(
-        `High-entropy paragraph detected (${entropy.toFixed(2)} bits/char) — possible encoded payload`
-      );
-    }
-  }
-  return { clean: findings.length === 0, findings };
-}
+// NOTE (#2198): scanEntropyAnomalies + shannonEntropy were removed as dead exports.
+// They had zero production callers — the live hooks (gsd-prompt-guard.js,
+// gsd-read-injection-scanner.js) inline their own pattern subsets for hook
+// independence and never called these functions. scanForInjection is retained
+// below: it serves as the CI codebase-scanner engine
+// (tests/prompt-injection-scan.security.test.cjs), not as a live hook.