ci: consolidate llama-cpp-darwin into the matrix-driven Darwin flow (#9731)

The bespoke llama-cpp-darwin + llama-cpp-darwin-publish top-level jobs
in backend.yml ran unconditionally on every backend.yml trigger
(push/cron), bypassing the path filter that all 34 other Darwin
backends already honor via backend-jobs-darwin -> backend_build_darwin.yml.

Move llama-cpp into the includeDarwin matrix:
- New entry in .github/backend-matrix.yml (lang=go, no build-type).
- backend_build_darwin.yml gains an `if: inputs.backend == 'llama-cpp'`
  build step that drives `make backends/llama-cpp-darwin`. The bespoke
  script (scripts/build/llama-cpp-darwin.sh) compiles three CMake
  variants from backend/cpp/llama-cpp and bundles dylibs via otool, so
  it doesn't fit the build-darwin-go-backend mold; the existing
  llama-cpp-aware ccache setup blocks already in this workflow are
  what motivated the consolidation in the first place.
- scripts/changed-backends.js's inferBackendPathDarwin gains a special
  case so llama-cpp on Darwin maps to backend/cpp/llama-cpp/ (the C++
  source tree) rather than the non-existent backend/go/llama-cpp/.
- Bumps Darwin go-version from 1.24.x -> 1.25.x in backend.yml and
  backend_pr.yml so llama-cpp keeps the Go toolchain it had under the
  bespoke job; the other 34 Darwin backends pick this up too with no
  known reason to pin 1.24.
- Removes ~80 lines of bespoke YAML from backend.yml.

The publish path is unchanged in shape - every Darwin backend now uses
the same crane-push leg from ubuntu-latest in
backend_build_darwin.yml; only the build target differs per backend.

After this commit, llama-cpp-darwin only rebuilds when
backend/cpp/llama-cpp/ is touched (verified locally) - same behavior
as every other Darwin backend.

Assisted-by: Claude:claude-opus-4-7

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>