feat(llm-security-copilot): port llm-security v5.1.0 to GitHub Copilot CLI

Full port of llm-security plugin for internal use on Windows with GitHub
Copilot CLI. Protocol translation layer (copilot-hook-runner.mjs)
normalizes Copilot camelCase I/O to Claude Code snake_case format — all
original hook scripts run unmodified.

- 8 hooks with protocol translation (stdin/stdout/exit code)
- 18 SKILL.md skills (Agent Skills Open Standard)
- 6 .agent.md agent definitions
- 20 scanners + 14 scanner lib modules (unchanged)
- 14 knowledge files (unchanged)
- 39 test files including copilot-port-verify.mjs (17 tests)
- Windows-ready: node:path, os.tmpdir(), process.execPath, no bash

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

plugins/llm-security-copilot/hooks/scripts/lib/cache-dir.mjs

@@ -0,0 +1,19 @@
+// cache-dir.mjs — Cross-platform cache directory resolution.
+// Windows: %LOCALAPPDATA%/llm-security or %APPDATA%/llm-security
+// macOS/Linux: ~/.cache/llm-security (XDG_CACHE_HOME fallback)
+
+import { homedir } from 'node:os';
+import { join } from 'node:path';
+
+export function getCacheDir() {
+  if (process.platform === 'win32') {
+    const localAppData = process.env.LOCALAPPDATA;
+    if (localAppData) return join(localAppData, 'llm-security');
+    const appData = process.env.APPDATA;
+    if (appData) return join(appData, 'llm-security');
+    return join(homedir(), '.cache', 'llm-security');
+  }
+  const xdg = process.env.XDG_CACHE_HOME;
+  if (xdg) return join(xdg, 'llm-security');
+  return join(homedir(), '.cache', 'llm-security');
+}