feat: allow explicit package-manager selection and inject resolved PM into init pipeline

[youaresoyoung]

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Summarize your changes:

ref

• electron-forge-docs #240

Summary

This PR improves support for alternative package managers (pnpm, Yarn Berry) during project initialization.
Previously, npx create-electron-app resolved to npm at init time (because lockfiles don’t exist yet and npx sets the npm user agent). As a result, templates depending on pnpm/yarn–specific setup were silently misconfigured.

This change allows users to explicitly choose a package manager via CLI prompt or --package-manager flag, and ensures that decision is consistently propagated through the init pipeline.

Problem

• Init-time mismatch: New projects were bootstrapped with npm, even if the user intended to use pnpm or Yarn Berry.
• Silent divergence: Switching to another PM afterward (e.g. running pnpm install) did not re-run template logic (e.g. .npmrc, pnpm.onlyBuiltDependencies).
• Root cause:
  • resolvePackageManager() relied on user-agent or existing lockfile. Neither works at init time.
  • Templates redundantly re-called resolvePackageManager(), diverging from the CLI’s decision.
  • Some setup depended on postinstall, which doesn’t run if the PM changes later.
• ref: feat(core): pnpm support #3822, feat: improve Yarn Berry support #3997

Solution

• Added explicit package manager selection during init:
  • CLI prompt (interactive)
  • --package-manager flag (non-interactive / CI)
• Modified resolvePackageManager() to accept and cache an explicit argument.
• Propagated the selected PM through init → template, avoiding re-detection.
• Added test coverage for explicit PM caching and fallback behavior.

Alternatives Considered

• Environment variable (FORGE_PACKAGE_MANAGER): simple, but too implicit.
• Lockfile bundling: brittle and introduces drift.
• Postinstall-driven setup: fragile and increasingly restricted in PM policies.
  Explicit selection + propagation was chosen for clarity and determinism.

Impact

• Deterministic init-time package manager choice.
• Consistent behavior between CLI and template layers.
• Improved pnpm and Yarn Berry compatibility without breaking default npm flow.

Result / Outcome

• When pnpm is selected, Forge now automatically adds:
  • npmrc file
  • pnpm.onlyBuiltDependencies configuration
• When yarn is selected (running with Yarn Berry), Forge now automatically adds:
  • .yarnrc file

This allows users to run Electron Forge out-of-the-box with their chosen package manager, without requiring any manual configuration.

Document

• https://www.notion.so/sozerojung/feat-init-allow-explicit-package-manager-selection-and-inject-resolved-PM-into-init-pipeline-fix--26cf33a18794807e9808e47dadf28d29?source=copy_link