Use @microsoft/vally library directly instead of vally-cli subprocess

Replace the npx-spawned vally-cli process with a direct call to the
@microsoft/vally core library in the external plugin quality gates scripts:

- Add @microsoft/vally as a devDependency in package.json
- Import runLint and LintConsoleReporter from @microsoft/vally
- Replace runVallyLintGate() process spawn with async API call:
  - runLint({ rootPath }) returns structured LintResults
  - LintConsoleReporter with a Writable capture stream collects
    text output without printing to stdout
- Make runExternalPluginQualityGates() async (propagated to
  runExternalPluginPrQualityGates() and both main entry points)
- Use Promise.all in runExternalPluginPrQualityGates() for parallel
  plugin checks
- Fix remaining skill_validator_status reference in pr-quality-gates
  summary string (now vally-lint=...) and YAML workflow table header
- Add 'npm install @microsoft/vally' step to both calling workflows

This removes a layer of indirection (Node -> npx -> CLI -> library)
and replaces it with a direct in-process library call, which is faster,
more reliable, and gives structured access to lint results.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/workflows/external-plugin-pr-quality-gates.yml

@@ -95,6 +95,9 @@ jobs:
       - name: Install GitHub Copilot CLI
         run: npm install -g @github/copilot
 
+      - name: Install @microsoft/vally
+        run: npm install @microsoft/vally
+
       - name: Run external plugin PR quality gates
         id: quality
         env:
@@ -205,7 +208,7 @@ jobs:
                   const sourceUrl = String(entry?.source_tree_url || '');
                   const locator = String(entry?.source?.sha || entry?.source?.ref || 'repository');
                   const sourceCell = sourceUrl ? `[${locator}](${sourceUrl})` : locator;
-                  return `| ${name} | ${quality.skill_validator_status || 'not_run'} | ${quality.smoke_status || 'not_run'} | ${quality.overall_status || 'not_run'} | ${sourceCell} |`;
+                  return `| ${name} | ${quality.vally_lint_status || 'not_run'} | ${quality.smoke_status || 'not_run'} | ${quality.overall_status || 'not_run'} | ${sourceCell} |`;
                 })
               : ['| _none_ | not_run | not_run | not_run | _n/a_ |'];
 
@@ -218,7 +221,7 @@ jobs:
               '',
               '### Per-plugin quality summary',
               '',
-              '| Plugin | skill-validator | install smoke test | overall | source tree |',
+              '| Plugin | vally lint | install smoke test | overall | source tree |',
               '|---|---|---|---|---|',
               ...rows,
               '',