ci: add autonomous CI/CD pipeline with Buildroot build and QEMU tests

[gustavosbarreto]

Summary

• Replace standalone update-hash.yml with a unified ci.yml pipeline
• Build Buildroot image with ShellHub agent and test it in QEMU automatically
• Auto-approve and squash-merge Renovate PRs after all checks pass

Pipeline

Renovate PR → update-hash → build-and-test → auto-merge
Manual PR   →               build-and-test

Jobs

1. update-hash (Renovate only): downloads tarball, recalculates MD5, commits if changed. Early-exits when hash is already correct to prevent double-runs from the push event.
2. build-and-test: clones Buildroot 2024.11, configures qemu_x86_64_defconfig + ShellHub + ccache, builds with make -j$(nproc), then boots in QEMU and validates:
   • /usr/bin/shellhub-agent exists and is executable
   • shellhub-agent --version runs successfully
   • /etc/init.d/S42shellhub init script is installed
3. auto-merge (Renovate only): auto-approves and squash-merges after tests pass.

Caching

Cache	Key strategy
dl/ (source tarballs)	buildroot-dl-{hash(shellhub.mk)}
ccache	buildroot-ccache-{run_id} (rolling)

Test plan

• This PR triggers build-and-test (no hash update or auto-merge since it's not from Renovate)
• Verify caches are created after first run
• Next Renovate PR should run the full pipeline including auto-merge