fix(node-vfs): rewrite absolute VFS paths in execute() commands

Summary

• Fix VfsSandbox.execute() to support absolute paths (e.g. node /src/index.js) by rewriting them to the temp directory path before shell execution
• Add integration tests covering absolute path command execution for single-file and multi-file Node.js projects

Problem

When users define initialFiles with absolute paths like /src/index.js, the VFS correctly stores and syncs files to a temp directory for execution. However, commands referencing those files with absolute paths fail:

const sandbox = await VfsSandbox.create({
  initialFiles: { "/src/index.js": "console.log('Hello')" },
});

await sandbox.execute("node src/index.js");   // ✅ works (relative → resolves via cwd)
await sandbox.execute("node /src/index.js");  // ❌ fails (absolute → resolves against host root)

The #normalizeExecPath method already handles leading / for read(), ls(), grep(), glob(), but execute() passes the raw command string directly to /bin/bash -c without any path translation.

Solution

Add a #rewriteVfsPaths(command, execDir) private method that:

1. Reads the top-level entry names from the VFS workspace (e.g. src, data.json)
2. For each entry, uses a regex to match /<name> at path-token boundaries in the command
3. Replaces matched absolute paths with <tempDir>/<name>

The regex uses negative lookbehind ((?<![\w/.-])) and positive lookahead ((?=/|[\s"';|&><!]|$)) to ensure only standalone absolute path tokens are rewritten, leaving system paths like /bin/bash or /usr/local/bin/node untouched (since bin, usr are not VFS entries).

Test plan

• New test: should run a multi-file Node.js project with absolute paths — verifies node /src/index.js with cross-file require()
• New test: should access initialFiles with leading slash via execute() using absolute path — verifies single-file absolute path execution
• New test: should run a multi-file Node.js project with absolute paths from README (absolute execute) — mirrors README example with absolute path
• All 28 unit tests pass
• All 125 integration tests pass (including 92 standard tests)