Creating backend abstraction for self hosted worker

[ianhodge]

What

Pure refactor: introduces a Backend interface and extracts all Docker logic from worker.go into a separate DockerBackend implementation. No behavior change.

Changes

• internal/worker/backend.go — New Backend interface with ExecuteTask and Shutdown methods
• internal/worker/docker.go — DockerBackend struct with all Docker-specific logic extracted from worker.go (image pulling, sidecar management, container lifecycle, registry auth, etc.)
• internal/worker/worker.go — Slimmed down: Worker struct now holds a backend Backend field instead of dockerClient/platform. New() creates a DockerBackend, executeTask delegates to w.backend.ExecuteTask(), Shutdown calls w.backend.Shutdown()

Testing

• Builds clean (go build ./...)
• Manually verified against staging — worker connects, receives task assignments, and executes them in Docker identically to before

Overall plan: non-containerized self-hosted agents

This PR is 1 of 3 to add direct (non-Docker) execution support:

1. PR 1 (this PR): Backend interface + Docker extraction — pure refactor to enable the next two PRs
2. PR 2: YAML config file support (--config-file) — adds gopkg.in/yaml.v3, config parsing/merging for worker_id, cleanup, docker.volumes, docker.environment
3. PR 3: Direct backend (--backend direct) — runs agents directly on the host via the Oz CLI, with setup/teardown scripts, per-agent workspace directories, and environment file injection

Co-Authored-By: Oz oz-agent@warp.dev

[ianhodge]

• Creating backend abstraction for self hosted worker  #29 👈 (View in Graphite)
• main

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[bnavetta]

I think we should have a slight layer of abstraction between the TaskAssignmentMessage and the backend interface.

There are a couple common concerns we shouldn't have to handle in the backend implementations:

• Resolving common environment variables, like the Git config below
• Choosing a default Docker image (even if some backends will ignore this)
• Determining the base CLI args (even if the full command is backend-specific)