Add a run-server skill for launching + browser-driving the FastAPI app

This sandbox ships with no Python/Node/Docker/browser and no sudo, so
the bulk of this is setup.sh: bootstrap Python via uv, then get
Playwright's Chromium (and tmux, also missing) working by extracting
their .deb dependencies non-root instead of apt-get install. driver.py
is a small Playwright REPL standing in for chromium-cli, which isn't
available here either.

Also carves out .claude/skills/ from the blanket .claude/ gitignore --
skills are shared project tooling, not personal/local state.

server/.claude/skills/run-server/stop-server.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+# Usage: ./stop-server.sh [scratch-dir]
+SCRATCH="${1:-/tmp/run-server-scratch}"
+if [ -f "$SCRATCH/server.pid" ]; then
+  kill "$(cat "$SCRATCH/server.pid")" 2>/dev/null || true
+  rm -f "$SCRATCH/server.pid"
+  echo "stopped"
+else
+  echo "no $SCRATCH/server.pid -- nothing to stop"
+fi