chore: scaffold reddit-post Playwright sidecar project

sidecars/reddit-post/README.md

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+# Reddit Post Sidecar
+
+A Playwright acceptance test that logs into Reddit with supplied credentials
+and creates a text post in a given subreddit.
+
+## One-time setup
+
+    cd sidecars/reddit-post
+    npm install
+    npx playwright install chromium
+
+## Usage
+
+    ./run.sh --username=YOUR_USERNAME --password=YOUR_PASSWORD \
+      --subreddit=test --title="Hello" --body="Post body text" [--headed]
+
+Prints a single JSON line to stdout and exits 0 on success, exits 1 on
+failure:
+
+    {"success": true, "url": "https://www.reddit.com/r/test/comments/.../"}
+    {"success": false, "error": "Login did not complete — possible CAPTCHA or 2FA prompt"}