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.

Resolved on main by the package.json delegation checkpoint 4d1b539e7 and failure-semantics follow-up 3afa6ccd1.

Pantry now treats a conventional package.json dependency graph as owned by its selected JavaScript package manager and delegates it exactly once, while explicit Pantry/system dependencies stay in the native Pantry pipeline. The follow-up closes the dangerous success-on-failure case: a missing package-manager executable, spawn error, signal termination, or non-zero Bun/pnpm/Yarn/npm exit now makes pantry install non-zero on fast-path, ordinary-project, and workspace installs. The .pantry-js-installed marker is written only after a successful child install.

Verification on Darwin:

• Pantry Zig suite: 36/36 build steps succeeded.
• New unit regression uses a project-local fake npm that exits 42, observes JsInstallFailed, and proves no marker is written.
• Real Pantry CLI black-box control: failing delegated npm exits 1; successful delegated npm exits 0 and writes the marker.
• Home's user-facing install delegation preserves the same 1/0 result and marker boundary, confirming Home reuses Pantry rather than adding another user-facing package-manager path.
• Documentation contracts: 4 pass / 0 fail; zig fmt --check and git diff --check passed.

This resolves the issue's npm-dependency installation contract. Broader layout and resolver questions remain separately tracked in #201.