Add screenshot capture tool and dev setup docs

docs/screenshots.md

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+# Screenshots
+
+`tools/screenshot` captures pages from the local dev server as PNGs, so humans and agents can verify visual changes. It drives a locally installed Chrome (or Chromium) headless via chromedp; no other browser tooling is required.
+
+## Usage
+
+With the server running:
+
+    go run ./tools/screenshot -url http://localhost:8080/directory/ -out screenshots/directory.png -wait header
+
+Flags:
+
+- `-url` — page to capture (default `http://localhost:8080/directory/`)
+- `-out` — output PNG path (default `screenshots/capture.png`); `screenshots/` is gitignored
+- `-wait` — CSS selector that must be visible before capture (default `body`); pass a selector the page's JavaScript renders (for example `.card`) to capture after data loads
+
+The capture is a full-page screenshot at a 1280×800 viewport.
+
+## Agent recipe
+
+One self-contained command that starts the server, captures, and shuts down:
+
+    go run . &
+    go run ./tools/screenshot -out screenshots/directory.png -wait header
+    kill $(lsof -ti :8080)
+
+Then read `screenshots/directory.png` to inspect the result.