v1.91.1.0 fix: harden Impeccable plugin discovery (#2978)

* fix(design-detect): find impeccable installed as a Claude Code plugin

The design-detector probe only ever checked <root>/<SKILL_ROOTS>/skills/impeccable/,
never the Claude Code plugin-cache layout
(<root>/.claude/plugins/cache/<marketplace>/<plugin>/<version>/skills/impeccable/).
A plugin-installed impeccable was therefore invisible: IMPECCABLE_SKILL stayed
absent, the launcher was never found (so the NOT_CACHED run hint never fired),
and its sibling engine was never considered.

Add a plugin-cache walk alongside the existing SKILL_ROOTS walk, sharing the
same presence/launcher/repo-local-exclusion/sibling-engine logic via an
extracted checkSkillDir() helper so both paths stay behaviorally identical.

Fixes #2838

* refactor(design-detect): consolidate newestSemverDir onto safeReaddir

Both did the identical try/catch-around-readdirSync; newestSemverDir now
reuses the new safeReaddir helper instead of duplicating it.

* fix: harden Impeccable plugin discovery and regression fixtures

* test: supply eval mode to the integrated detector callback adapter

---------

Co-authored-by: Som Samantray <som.samantray@gmail.com>

test/fake-impeccable-touchfiles.test.ts

@@ -8,6 +8,7 @@ describe('fake impeccable engine selection', () => {
     'design-html-slop-gate',
     'design-review-detector-shim',
     'design-review-detector-shim-dom',
+    'design-review-plugin-handoff',
     'review-design-lite',
   ];
 
@@ -28,6 +29,7 @@ describe('fake impeccable engine selection', () => {
       ['design-html-slop-gate', 'periodic'],
       ['design-review-detector-shim', 'gate'],
       ['design-review-detector-shim-dom', 'gate'],
+      ['design-review-plugin-handoff', 'gate'],
       ['review-design-lite', 'periodic'],
     ]);
   });