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/docs/security-hardening-guide.md

@@ -192,11 +192,13 @@ suppression layers:
    .css, .scss, .sass, .less, .svg` + compound `.min.js, .min.css, .map`. A
    skip counter (`calibration.files_skipped_by_extension`) is reported in the
    scanner envelope.
-2. **Line-level rules 11–17** — applied when a line contains any of: GLSL
+2. **Line-level rules 11–18** — applied when a line contains any of: GLSL
    keywords (`uniform`, `vec3`, `texture2D`…), CSS-in-JS templates
    (`styled.…`), inline `<svg>` markup, ffmpeg `filter_complex` syntax,
    browser `User-Agent` strings, SQL DDL on a dedicated line
-   (`^\s*(SELECT|INSERT|…)`), or `throw new Error(\`…\`)` templates.
+   (`^\s*(SELECT|INSERT|…)`), `throw new Error(\`…\`)` templates, or
+   markdown image syntax with external URL (`![alt](https://cdn…)` — common
+   in JSON content indexes / article metadata).
 3. **Per-project policy override** — `.llm-security/policy.json` `entropy`
    section supports: