github: add LLM PR review wrapper and rules

github: add LLM PR review wrapper and rules

Drop-in wrapper that calls the reusable LLM review workflows in
openwrt/actions-shared-workflows. Triggers on pull_request_target
(incl. PRs from forks), a nightly cron (03:00 UTC), and manual
workflow_dispatch with a max_prs input override.

A detect-kernels pre-step builds the extra_repos list at workflow
runtime: it reads target/linux/generic/kernel-* in the base branch
to derive gregkh/linux:v<X.Y.Z> tags for each currently-targeted
kernel, then appends u-boot/u-boot:master. The list updates
automatically when kernel versions are bumped; the routine clones
only the entries actually needed for a given diff.

The bundled .github/llm-review-rules.md teaches the bot two
project-specific deprecations to flag even when other in-tree files
still use the legacy form:

  - LED label syntax `label = "<color>:<function>";` -->
    `color = <LED_COLOR_ID_*>;` + `function = "<func>";`
  - `mediatek,mtd-eeprom` for MAC sourcing -->
    `nvmem-cells` + `nvmem-cell-names = "mac-address";`

Repository settings need LLM_ROUTINE_ID_PR / LLM_ROUTINE_TOKEN_PR
and the *_NIGHTLY counterparts populated before the workflow can
fire. See openwrt/actions-shared-workflows/docs/llm-review-setup.md
for the full setup procedure.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Link: https://github.com/openwrt/openwrt/pull/23105
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>