fix(smolmachines): run claude mcp add as node so config lands in node's home

provision_supervise dispatched `claude mcp add --scope user`
through `smolvm machine_exec`, which runs as root by default.
The MCP entry got written to root's ~/.claude.json — but the
agent's claude reads /home/node/.claude.json, so `/mcp` showed
"No MCP servers configured" inside the bottle.

Wrap the exec in `runuser -u node -- env HOME=/home/node ...`
so the config writes to the right home. Same pattern as the
interactive exec_claude / Bottle.exec wrappers — `smolvm
machine_exec` is always root, so any command that touches user
state has to switch UID + set HOME explicitly.

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

tests/unit/test_smolmachines_provision.py

@@ -487,11 +487,15 @@ class TestProvisionSupervise(unittest.TestCase):
             _supervise.provision_supervise(plan, "claude-bottle-demo-abc12")
         ex.assert_called_once()
         argv = ex.call_args.args[1]
-        # claude mcp add --scope user --transport http supervise <url>
-        # — URL is the agent-side endpoint (host loopback +
-        # discovered port), not the docker bridge IP.
+        # `claude mcp add --scope user` writes to ~/.claude.json,
+        # and the agent is the `node` user — switch UID + set
+        # HOME so the config lands in /home/node/.claude.json,
+        # not root's. URL is the agent-side endpoint (host
+        # loopback + discovered port), not the docker bridge IP.
         self.assertEqual(
             [
+                "runuser", "-u", "node", "--",
+                "env", "HOME=/home/node",
                 "claude", "mcp", "add",
                 "--scope", "user",
                 "--transport", "http",