Remove docker-runner deployment artifacts from monorepo after extraction to agynio/docker-runner

[rowan-stein]

User Request

Complete the extraction of docker-runner into agynio/docker-runner by removing deployment artifacts from this monorepo. The packages/docker-runner/ workspace package stays — it's used as a shared library by platform-server.

Specification

What to REMOVE

1. packages/docker-runner/Dockerfile — image is now built from agynio/docker-runner
2. charts/docker-runner/ — Helm chart is now in agynio/docker-runner
3. .github/workflows/docker-ghcr.yml — remove docker-runner from the build matrix
4. .github/workflows/release.yaml — remove docker-runner from the images matrix and from the CHARTS env var

What to UPDATE

5. README.md — add note that docker-runner is now built and deployed from agynio/docker-runner

What STAYS (do NOT remove)

• packages/docker-runner/ (source, tests, package.json, tsconfig, eslint config) — workspace dependency for platform-server
• buf.gen.yaml entries for docker-runner — still needed for proto generation
• All @agyn/docker-runner imports in platform-server

Acceptance Criteria

• pnpm install works
• pnpm proto:generate works
• pnpm lint passes
• pnpm test passes
• No Dockerfile in packages/docker-runner/
• No charts/docker-runner/ directory
• CI workflows no longer build docker-runner images