ci: publish Docker images with dev tag on push to dev branch

Problem

Currently Docker images are only built and published on release tags (v*.*.*). The dev branch has no Docker image, making it hard for testers and developers to run the latest unreleased version without building locally.

Solution

Add a CI workflow that builds and pushes Docker images tagged dev, dev-base, dev-full, dev-otel on every push to the dev branch. This mirrors the release matrix (base/latest/full/otel variants) but uses dev prefix instead of version numbers.

Registries: GHCR + Docker Hub (same as release)
Platforms: linux/amd64, linux/arm64
Trigger: push to dev branch

Usage

docker pull digitop/goclaw:dev        # latest dev build
docker pull digitop/goclaw:dev-full   # dev with all runtimes

Acceptance Criteria

• Docker images built on every push to dev
• All 4 variants published (dev, dev-base, dev-full, dev-otel)
• Both GHCR and Docker Hub registries
• Multi-arch (amd64 + arm64)
• GHA cache enabled for fast rebuilds

Start by locating the existing CI workflow that builds and publishes release-tag Docker images, then compare its variant, registry, platform, and cache matrix. Add the dev-branch trigger and matching dev tags, and verify that pushes publish all four variants to GHCR and Docker Hub for amd64 and arm64.