feat(llm-security): v7.0.0 commit 7 — rule 18 (markdown image URL suppression)

E2E verification against content-heavy repo (`content-claude-code`) revealed
413 entropy findings (8 HIGH / 405 MEDIUM) from markdown image CDN URLs in
JSON content indexes — e.g., `![Image 1: Title](https://www-cdn.anthropic.com/images/.../cf1dd2167fcf12f5882333ddc58a5bc1f0026952.svg)`.
These are legitimate content-repo artifacts, not credentials. The 40-char
hash segment in the CDN URL trips Shannon entropy (H=5.29 over 300 chars),
and rule 13 (inline <svg>) doesn't match since there's no literal `<svg>`
tag — the `.svg` is just a URL path suffix.

Added rule 18 `MARKDOWN_IMAGE = /!\[[^\]]*\]\(\s*https?:\/\//` — matches
`![alt](http…)` / `![alt](https…)`. Line-level (not string-level) so URL
is not over-specific.

E2E impact on `content-claude-code`:
- Before: BLOCK / 65 / 8H 437M 0L
- After:  WARNING / 56 / 3H 427M 0L

Hyperframes unchanged: BLOCK / 80 / 1C 4H 92M — real CRITICAL SQL-injection
and HIGH findings still detected.

Tests: 2 new (positive + negative fixture) bringing entropy-context to 26,
total suite 1485 → 1487.

Docs updated to "rules 11-18" and "8 new line-suppression rules".

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

plugins/llm-security/tests/scanners/entropy-context.test.mjs

@@ -186,6 +186,28 @@ describe('entropy-scanner context suppression (v7.0.0+)', () => {
       assert.equal(result.findings.length, 0, 'expected throw new Error line to suppress');
       await rm(fx, { recursive: true, force: true });
     });
+
+    it('rule 18: markdown image with external URL suppresses finding', async () => {
+      const fx = await newRoot('ent-rule18-');
+      await writeFixture(fx, 'index.json',
+        '{"summary": "![Image 1: Title](https://cdn.example.com/abc/' + PAYLOAD + '.svg)"}');
+      resetCounter();
+      const discovery = await discoverFiles(fx);
+      const result = await scan(fx, discovery);
+      assert.equal(result.findings.length, 0, 'expected markdown image line to suppress');
+      await rm(fx, { recursive: true, force: true });
+    });
+
+    it('rule 18 does NOT over-match plain URLs without image syntax', async () => {
+      const fx = await newRoot('ent-rule18b-');
+      await writeFixture(fx, 'app.js',
+        'const token = "' + PAYLOAD + '"; // not an image');
+      resetCounter();
+      const discovery = await discoverFiles(fx);
+      const result = await scan(fx, discovery);
+      assert.ok(result.findings.length >= 1, 'plain high-entropy string must still be detected');
+      await rm(fx, { recursive: true, force: true });
+    });
   });
 
   describe('C. Policy-driven overrides', () => {