github: add LLM review workflow and project rules

github: add LLM review workflow and project rules

Add a nightly LLM-driven PR review for the LuCI repo, calling the
reusable workflow in actions-shared-workflows. The extra_repos list
pre-clones LuCI's typical backends (rpcd, netifd, firewall4, ucode,
uhttpd, packages, openwrt) so the routine can verify cross-repo
references without re-cloning per PR.

Also add .github/llm-review-rules.md with two project-specific rules
the system prompt does not cover: redirecting PRs that touch
out-of-tree luci-app-* packages, and checking that new UCI options
have a backend consumer.

Co-Authored-By: Claude Opus 4.7 (1M context) <redacted>
Signed-off-by: Hauke Mehrtens <redacted>