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/.gitignore

@@ -0,0 +1,2 @@
+# Generated by setup.sh -- bakes in this host's /tmp paths, not portable.
+env.sh