docs: add update-flow docs, PlantUML diagram, developer guide

docs/developer-guide.md

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+<!-- Developer guide: local dev and test commands -->
+# Developer Guide — Running tests & race detector
+
+This short guide covers how to run unit tests locally and how to run the race-enabled test suite in a Linux container (recommended for Windows hosts).
+
+## Prerequisites
+
+- Go toolchain (version compatible with project go.mod). To run `go test` locally, ensure `go` is in your PATH.
+- Docker (for running a Linux container to execute `-race` with CGO enabled)
+- Optional: GitHub CLI `gh` to open PRs from the command line.
+
+## Run unit tests locally
+
+From the repository root:
+
+PowerShell
+
+```powershell
+go test ./... -v
+```
+
+If you only want to run a package tests, run:
+
+```powershell
+go test ./pkg/registry/auth -v
+```
+
+## Run race detector (recommended via container on Windows)
+
+The Go race detector requires cgo and a C toolchain. On Linux runners this is usually available; on Windows it's simplest to run tests inside a Linux container.
+
+Example (PowerShell):
+
+```powershell
+docker run --rm -v "${PWD}:/work" -w /work -e CGO_ENABLED=1 golang:1.20 bash -lc "apt-get update && apt-get install -y build-essential ; /usr/local/go/bin/go test -race ./... -v"
+```
+
+Notes:
+- The command mounts the current working directory into the container and installs `build-essential` to provide a C toolchain so `-race` works.
+- If you prefer a faster run, run `go test -run TestName ./pkg/yourpkg -race`.
+
+## Render PlantUML diagrams (local)
+
+To render PlantUML into SVG using Docker (no Java/PlantUML install required):
+
+```powershell
+docker run --rm -v "${PWD}:/work" -w /work plantuml/plantuml -tsvg docs/diagrams/update-flow.puml
+```
+
+Move the generated SVG into the docs assets folder:
+
+```powershell
+mkdir docs/assets/images -Force
+Move-Item docs/diagrams/update-flow.svg docs/assets/images/update-flow.svg -Force
+```
+
+## Create a branch and PR (example)
+
+Example git commands:
+
+```powershell
+git checkout -b docs/update-flow
+git add docs/update-flow.md docs/diagrams/update-flow.puml docs/developer-guide.md docs/assets/images/update-flow.svg
+git commit -m "docs: add update flow docs, diagrams and developer guide"
+git push -u origin docs/update-flow
+```
+
+If you have the GitHub CLI installed you can open a PR with:
+
+```powershell
+gh pr create --title "docs: update flow + diagrams" --body "Adds update flow documentation, a PlantUML diagram and developer guide." --base main
+```
+
+If `gh` is not installed you can open a PR via GitHub web UI after pushing the branch.
+
+---
+
+If you'd like, I can push the branch and attempt to open the PR for you now.