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/design-detector-source-fixture.test.ts

@@ -58,7 +58,7 @@ async function exercise(modes: Mode[], retention?: 'directory' | 'run-id' | 'bot
     },
   };
   const args = {
-    ROOT, fs: localFs, os: { tmpdir: () => scratch }, path, expect, CAPTURE_MS, CAPTURE_LONG_MS,
+    ROOT, fs: localFs, os: { tmpdir: () => scratch }, path, expect, CAPTURE_MS, CAPTURE_LONG_MS, evalsEnabled: true,
     process: { ...process, env: { ...process.env, ...env } }, resolveEvalModel,
     getProjectEvalDir: () => artifactRoot,
     console: { ...console, log: (...args: any[]) => notices.push(args.join(' ')), error: (...args: any[]) => notices.push(args.join(' ')) },