Upgrade package manager from pnpm 10 to pnpm 11

## Overview

This issue tracks the upgrade of the package manager from pnpm 10 to pnpm 11.

## pnpm 11 Release Notes

- **Blog post / release notes:** https://pnpm.io/blog/releases/11.0
- **Migration guide (v10 → v11):** https://pnpm.io/blog/releases/11.0#migrating-from-v10-to-v11
- **GitHub releases:** https://github.com/pnpm/pnpm/releases

## Key Changes in pnpm 11

- **Node.js 22+ required** — support for Node.js 18, 19, 20, and 21 is dropped. pnpm is now pure ESM.
- **Supply-chain protection on by default** — `minimumReleaseAge` defaults to 1 day (1440 min) and `blockExoticSubdeps` defaults to `true`.
- **`allowBuilds` replaces legacy build-dependency settings** — `onlyBuiltDependencies`, `neverBuiltDependencies`, `ignoredBuiltDependencies`, and `ignoreDepScripts` are removed.
- **Configuration split** — `.npmrc` is now auth/registry only; all other pnpm settings must live in `pnpm-workspace.yaml` or the new global `~/.config/pnpm/config.yaml`.
- **New SQLite-backed store index (store v11)** — replaces JSON-per-package index for faster installs.
- **Native publish flow** — `pnpm publish`, `login`, `logout`, etc. no longer delegate to npm CLI.
- **Isolated global installs** via the global virtual store.
- **New commands** — `pnpm ci`, `pnpm sbom`, `pnpm clean`, `pnpm peers check`, `pnpm runtime set`, and short `pn`/`pnx` aliases.
- **Environment variables** use `pnpm_config_*` prefix instead of `npm_config_*`.

## Required Changes in This Repository

1. Update `packageManager` field in `package.json` to `pnpm@11.x.x`.
2. Migrate `pnpm-workspace.yaml` — replace `allowBuilds: false` entries with the new `allowBuilds` model; note that `strictDepBuilds` and related legacy fields are removed.
3. Ensure CI environment uses Node.js 22+.
4. Review any `.npmrc` settings and move non-auth config to `pnpm-workspace.yaml`.
5. Update the lockfile (`pnpm-lock.yaml`) by running `pnpm install` after the upgrade.
6. Update `scripts/check-pm.js` if needed (the `npm_config_user_agent` env var logic may need updating since pnpm 11 uses `pnpm_config_*` prefix for its own settings).

## Context

This issue was raised in the context of PR #145, which introduced pnpm 10 as the package manager.

**Requested by:** @infeo