ci: split test and Docker workflows for independent maintenance

[Skulldorom]

Summary

Splits the monolithic CI workflow into two independent files:

.github/workflows/test.yml — runs on every PR and push to main. Tests only, no Docker.
.github/workflows/docker.yml — triggers via workflow_run only when tests pass on main. Builds and publishes the Docker image.

Design

Trigger	What runs
PR opened/updated	Tests only (fast feedback, no image build)
Push to main	Tests → if pass → Docker build+push
Push to other branches	Nothing (no wasted CI)

The Docker workflow uses workflow_run with conclusion == 'success' gating so the image is only published after verified tests on main.

Why

• Tests and Docker are now independently maintainable — no more editing the same file for both concerns
• Running tests on PR doesn't need to know about Docker infrastructure
• The two badge approach (test + Docker) gives visibility into both pipelines

README badges

[![Tests](.../test.yml/badge.svg)](...)
[![Docker](.../docker.yml/badge.svg)](...)

Note: the Docker badge will show "no status" until this PR is merged and the workflow_run triggers for the first time on main.