fix(acr): file-discovery skips backups/ dirs — never live config (M-BUG-8)

A directory named `backups` holds backup COPIES, not live config, so walking
it during a config audit produces stale findings. config-audit's own session
backups (~/.claude/config-audit/backups/<ts>/files/.../CLAUDE.md) were the
canonical case: a ~/.claude-scope audit walked 36 frozen config copies as if
live, polluting CPS (C3) and HKV/RUL (C6) results. Same non-live-noise family
as M-BUG-2.

- Add `backups` to SKIP_DIRS (broad, name-based — consistent with vendor/dist/
  .cache): the rule is general, a backups/ dir is never live config.

TDD: 2 failing tests (no config under backups/ discovered + backups/ counted
as skipped) -> green; a third asserts live config beside the backups tree is
still discovered. Full suite 1310/0 (+3). Byte-stable: no fixture is named
`backups` and `backups` appears in zero frozen snapshots, so v5.0.0 + SC-5 +
default-output outputs are unchanged. Dogfooding ~/.claude: files under
/backups/ drop from 36 -> 0; 717 live config files retained.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EnUvKEqyEa1m9gy6Aqhdqq

scanners/lib/file-discovery.mjs

@@ -11,6 +11,11 @@ const SKIP_DIRS = new Set([
   'node_modules', '.git', 'dist', 'build', 'coverage', '__pycache__',
   '.next', '.nuxt', '.output', '.cache', '.turbo', '.parcel-cache',
   'vendor', 'venv', '.venv', '.tox',
+  // A `backups` dir holds backup COPIES, not live config — auditing it as if
+  // live produces stale findings. config-audit's own session backups
+  // (~/.claude/config-audit/backups/<ts>/files/.../CLAUDE.md) are the canonical
+  // case (M-BUG-8), but the rule is general: backups are never live config.
+  'backups',
 ]);
 
 // Path marker for the plugin install cache (~/.claude/plugins/cache).