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/periodic-fixture-selection.test.ts

@@ -828,7 +828,7 @@ test('stderr lifecycle regression selects runtime consumers without a quality-ma
     'outside-voice-codex-to-claude-code', 'outside-voice-claude-code-to-codex', 'outside-plan-disabled-no-fallback', 'ship-coverage-audit', 'review-coverage-audit',
     'plan-eng-coverage-audit', 'ship-triage', 'ship-docsync', 'docsync-spawned', 'design-consultation-core',
     'design-consultation-existing', 'design-consultation-research', 'design-consultation-preview', 'plan-design-review-no-ui-scope', 'design-review-fix',
-    'design-review-detector-shim', 'design-review-detector-shim-dom', 'design-html-slop-gate', 'diagram-triplet', 'diagram-authoring-quality',
+    'design-review-detector-shim', 'design-review-detector-shim-dom', 'design-review-plugin-handoff', 'design-html-slop-gate', 'diagram-triplet', 'diagram-authoring-quality',
     'gstack-upgrade-happy-path', 'land-and-deploy-workflow', 'land-and-deploy-first-run', 'land-and-deploy-review-gate', 'canary-workflow',
     'benchmark-workflow', 'setup-deploy-workflow', 'autoplan-dual-voice', 'scrape-match-path', 'scrape-prototype-path',
     'skillify-happy-path', 'skillify-provenance-refusal', 'skillify-approval-reject', 'journey-ideation', 'journey-plan-eng',