No shell available. Mount a WasmVM runtime to enable exec()

[ishaksebsib]

when running the example in the README

import { createKernel, createInMemoryFileSystem, createNodeRuntime } from "secure-exec";

const kernel = createKernel({
  filesystem: createInMemoryFileSystem(),
});
await kernel.mount(createNodeRuntime());

const result = await kernel.exec("node -e \"console.log('hello from secure-exec')\"");
console.log(result.stdout); // "hello from secure-exec\n"

await kernel.dispose();

I get:

236 |         // Flush pending /bin stubs before shell PATH lookup
237 |         await this.flushPendingBinEntries();
238 |         // Route through shell
239 |         const shell = this.commandRegistry.resolve("sh");
240 |         if (!shell) {
241 |             throw new Error("No shell available. Mount a WasmVM runtime to enable exec().");
                            ^
error: No shell available. Mount a WasmVM runtime to enable exec().

[aayushprsingh]

Hey! Ran into issue #64 when trying the README example — kernel.exec('node -e "console.log(1)"') throws immediately because createNodeRuntime() registers 'node', not 'sh'.

This PR adds a fallback: when sh isn't registered but node is, we parse the command string, strip the node prefix, and spawn node directly. Also extracted #collectExecResult() to avoid duplication between the two paths, and added a shell-like arg tokenizer for the node fallback.

Happy to iterate if you'd like a different approach — e.g. if you'd prefer to just improve the error message rather than auto-fallback. Let me know!