feat(nodejs): add pnpm support with auto-detection in Node.js plugin

[bruno-archastro]

This PR adds support for pnpm alongside npm in the Node.js plugin, with automatic detection of the package manager used by each project. Key changes:

• Introduces a PackageManager enum as the single source of truth for all npm/pnpm command strings and install lock resources.
• Implements detect_package_manager, which walks up from the project directory to the workspace root, preferring the packageManager field in package.json, then falling back to lockfiles (pnpm-lock.yaml → pnpm, package-lock.json → npm), defaulting to npm if nothing matches.
• All generated commands (install, test, run, exec) now use the detected package manager.
• Workspace members inherit the package manager from their workspace root.
• Updates the README to document pnpm support and detection logic.
• Adds comprehensive tests for:
  • Detection via packageManager field and lockfiles
  • Per-manager install lock resource
  • pnpm exec for tools like Prettier
  • Workspace inheritance of the package manager

Existing behavior is preserved for projects without a lockfile or packageManager field (defaults to npm). This enables seamless support for both npm and pnpm workflows.

[archastro]

✅ Clean PR, @bruno-archastro! No blocking findings on f2ac019 — a few nice things I clocked:

• Nice move centralizing every emitted command in the PackageManager enum — install/test/run/exec/cache_resource all flow through one source of truth instead of scattered literals. 🎯
• Clean layered detection in detect_package_manager — closest-wins packageManager field, then lockfile fallback, then npm default, with workspace members inheriting the root by walking up to workspace_root.
• Solid red-green discipline — every new mode is locked in by a test (field detection, lockfile fallback, pnpm_cache resource isolation, pnpm exec, and workspace inheritance). 🙌

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