feat(config-audit): flag additionalDirectories > 2 (v5 M6) [skip-docs]

- Add 'additionalDirectories' to KNOWN_KEYS
- Emit low severity finding when length > 2
- New fixtures: additional-dirs-many (3 entries) + additional-dirs-ok (2)

569 → 572 tests, all green.

plugins/config-audit/tests/fixtures/additional-dirs-many/.claude/settings.json

@@ -0,0 +1,8 @@
+{
+  "$schema": "https://json.schemastore.org/claude-code-settings.json",
+  "additionalDirectories": [
+    "~/work/repo-a",
+    "~/work/repo-b",
+    "~/work/repo-c"
+  ]
+}

plugins/config-audit/tests/fixtures/additional-dirs-ok/.claude/settings.json

@@ -0,0 +1,7 @@
+{
+  "$schema": "https://json.schemastore.org/claude-code-settings.json",
+  "additionalDirectories": [
+    "~/work/repo-a",
+    "~/work/repo-b"
+  ]
+}