pantry install should also install npm dependencies (bun install)

Problem

When migrating CI workflows from bun install to pantry install, npm dependencies are not installed. This causes builds to fail with errors like:

error: Could not resolve: "@stacksjs/clapp". Maybe you need to "bun install"?
error: Could not resolve: "ts-syntax-highlighter". Maybe you need to "bun install"?
error: Could not resolve: "bunfig". Maybe you need to "bun install"?

Current behavior

pantry install handles:

• System packages (bun runtime, zig, etc.) from pantry.jsonc/deps.yaml
• Workspace packages placed in pantry/
• Lockfile management via pantry.lock
• PHP deps via Composer delegation
• Zig deps via build.zig.zon sync
• Pre/post install hooks

But it does not install npm dependencies from package.json dependencies/devDependencies. These packages live in node_modules/ and are managed by bun install.

Expected behavior

pantry install should be a complete replacement for bun install in CI workflows. When a project has a package.json with npm dependencies, pantry install should also run bun install (or equivalent) to populate node_modules/.

This would allow CI workflows to simply use:

- name: Setup Pantry
  uses: home-lang/pantry/packages/action@main

- name: Install Dependencies
  run: pantry install

Instead of needing both:

- name: Setup Pantry
  uses: home-lang/pantry/packages/action@main

- name: Install Dependencies
  run: bun install

Context

We attempted to migrate 3 repos (bunpress, craft, dtsx) from bun install to pantry install in CI. All builds failed because npm packages weren't installed.

The install logic in packages/zig/src/cli/commands/install/core.zig does not delegate to bun install for npm dependencies. It handles pantry registry packages, Composer delegation for PHP, and Zig deps — but no npm/bun delegation.

Suggested implementation

After the pantry-specific install steps complete, detect if package.json exists with npm dependencies and run bun install automatically. Similar to how Composer delegation already works in install/core.zig:981-989.

// Pseudocode for the additional step:
if (fileExists("package.json") && hasNpmDependencies()) {
    runCommand("bun install");
}

This would make pantry install a true superset that handles everything — system packages, language-specific packages, AND npm dependencies.