docs: add package-updater implementation plan
19-task TDD plan covering project scaffolding, config, structured logging, discovery (image matching, Compose label parsing, path safety, orchestration), executor abstraction, FIFO queue + single worker, HTTP middleware (auth, request_id, request logging), handlers (/update, /healthz, /version), Prometheus metrics, self-update deferred execution, wire-up in main, Dockerfile, example compose, Gitea composite action, and README.
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