Add CLAUDE.md with project guidance and update CI workflow for improved coverage and Docker build

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Commands
+
+```bash
+# Run all tests with coverage
+go test -v -coverprofile=coverage.out -coverpkg=./... ./...
+
+# Run a single test
+go test -v -run TestWebhookHandler ./test/diunwebhook/
+
+# Run the app locally
+go run ./cmd/diunwebhook/
+
+# Build Docker image
+docker build -t diun-webhook-dashboard .
+
+# Run with Docker Compose
+docker compose up -d
+```
+
+CI warns (but does not fail) when coverage drops below 80%.
+
+## Architecture
+
+The app is a minimal Go HTTP server that receives [DIUN](https://crazymax.dev/diun/) webhook events and exposes them via a JSON API and static HTML dashboard. All state is in-memory (no persistence).
+
+**Package layout:**
+- `pkg/diunwebhook/` — core library: `DiunEvent` struct, in-memory `updates` map (guarded by `sync.Mutex`), and HTTP handlers (`WebhookHandler`, `UpdatesHandler`)
+- `cmd/diunwebhook/main.go` — wires the handlers and static file server onto `net/http`'s default mux, listens on `:8080`
+- `test/diunwebhook/` — external test package (`package diunwebhook_test`) that imports `pkg/diunwebhook`; uses `httptest` for handler tests
+- `static/` — served verbatim at `/`
+
+**Key data flow:**
+1. DIUN POSTs JSON to `/webhook` → `WebhookHandler` decodes into `DiunEvent` → stored in `updates[event.Image]` (latest event per image wins)
+2. Dashboard JS polls `GET /api/updates` → `UpdatesHandler` returns full map as JSON
+
+**Test helpers exposed from the library package** (not part of the public API, only for tests): `GetUpdatesMap()`, `UpdatesReset()`, `UpdateEvent()`.