refactor: Bottle.exec takes a user= kwarg, default node

Promote the user-switch from a hardcoded `node` to a keyword arg
so callers can opt into root (or any other user) when needed.
Default stays `node` — matches the docker image's USER and the
smolmachines runuser default.

Lifts the change through the base ABC, docker, and smolmachines
backends:
- Base: `def exec(self, script, *, user="node")`.
- Docker: adds `-u <user>` to `docker exec` (no-op when user is
  node, the image's default).
- Smolmachines: `runuser -l <user> -c <script>` — `runuser -l
  root` is the trivial no-op form when the caller asked for root.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

claude_bottle/backend/smolmachines/bottle.py

@@ -65,15 +65,21 @@ class SmolmachinesBottle(Bottle):
         result = subprocess.run(flags, check=False)
         return result.returncode
 
-    def exec(self, script: str) -> ExecResult:
-        """Run a POSIX shell script as the `node` user and capture
-        the result. Matches the docker backend's `exec`, which
-        defaults to the image's USER (also node) — so test
+    def exec(self, script: str, *, user: str = "node") -> ExecResult:
+        """Run a POSIX shell script as `user` (default `node`) and
+        capture the result. Matches the docker backend's `exec`,
+        which defaults to the image's USER (also node) — so test
         helpers / provision shell-outs run with the same identity
-        on both backends."""
+        on both backends. Pass `user="root"` for tests that need
+        root.
+
+        `smolvm machine exec` runs commands as root in the VM, so
+        we always need to switch user (even when the caller asked
+        for root, switching to root is a cheap no-op via
+        `runuser -l root`)."""
         r = _smolvm.machine_exec(
             self.name,
-            ["runuser", "-l", "node", "-c", script],
+            ["runuser", "-l", user, "-c", script],
         )
         return ExecResult(
             returncode=r.returncode,