feat(ci): add /explain-ci bot command for PR failure help

Adds a new GitHub Actions workflow that responds to `/explain-ci`
comments on pull requests. When triggered, nextcloud-bot posts a
summary of all currently-failing CI checks with a plain-language
description of what each check validates and step-by-step local fix
instructions.

Covers all 40 developer-facing PR workflows: PHPUnit (all DB
backends), linting (ESLint, Stylelint, PHP, PHP-CS-Fixer), Psalm,
Cypress, Node build/tests, REUSE, OpenAPI, Rector, Behat integration
tests, external storage tests, object storage tests, block checks
(fixup commits, conventional commits, outdated 3rdparty), Code
checkers, and CodeQL.

Uses the same slash-command pattern as /compile and /update-3rdparty:
issue_comment trigger (works on fork PRs), COMMAND_BOT_PAT for bot
identity, and peter-evans/create-or-update-comment for the +1
reaction acknowledgement.

AI-Assisted-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Anna Larch <anna@nextcloud.com>

Author: miaulalala