ci: document the paths-ignore / required-check paradox

Add a comment in the workflow header explaining why docs-only PRs may
end up BLOCKED if `lint` / `test` are required status checks. The
proper fix is a status-check shim workflow with matching job names —
left for a follow-up.

(This commit also exists to force CI to run on a docs-only PR that
got stuck under exactly the situation it documents.)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: obiot

.github/workflows/main.yml

@@ -1,4 +1,9 @@
 name: Build & Test
+# Note: docs-only PRs (markdown / media changes) skip these jobs via
+# `paths-ignore`. If branch-protection rules require `lint`/`test` as
+# checks, those PRs may end up BLOCKED with no checks ever running. The
+# workaround is admin-merge (or a status-check shim emitting matching
+# job names with `success` for the same paths).
 on:
   push:
     branches: [master]