process: preflight does not fail a task branch for foreign causes (#700)

11 of 85 returns in #600–#691 were the thin-workflow mirror check, and any
push could turn red because a foreign site behind a docs link was down.

- validate.yml preflight: on refs/heads/issue/* the workflow_sync mismatch
  is a warning in the summary, not a failed verdict; push to dev, the beta
  candidate and the release keep it red.
- On push to dev a mismatch opens one owner issue titled [workflow-sync]
  (or comments on the open one), like the nightly mutation gate (#472);
  preflight gets issues: write for that.
- check-docs --external=warn: external link failures become warnings; the
  docs step passes it on task branches only.

Canon: PROCESS.md §10.4 («Workflow из ветки по умолчанию»).

Issue: #700
User-Visible: no
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018qZfe7YS4rqEMKoVeS3GKd

test/classify-changes.test.mjs

@@ -215,7 +215,7 @@ test('#586: preflight спрашивает режим одним значени
   assert.match(workflow, /classify-changes\.mjs --screenshots-mode/);
   assert.ok(!/=\s*"heavy=true"/.test(workflow),
     'сравнение со строкой «heavy=true» вернулось — строгий режим снова не включится');
-  assert.match(workflow, /check-docs\.mjs --external --screenshots=\$mode/);
+  assert.match(workflow, /check-docs\.mjs "\$external" --screenshots=\$mode/);
 });
 
 test('#510 AC1 / #601 AC1: мутанты по диффу запрашиваются только кнопкой mutants=true и PR — не пушем, не кандидатом беты, не full', () => {