fix: invoke pnpm via npm_execpath on Windows (fixes #19)

[Sainigurnoor511]

Summary

• Add \scripts/pnpm-command.mjs\ to resolve pnpm invocations on Windows without a shell: when
  pm_execpath\ points at pnpm's .cjs/.mjs\ entry (as under \pnpm run), run
  ode ...\ so argv boundaries survive intact.
• Reject
  pm-cli.js\ via the path guard so
  pm run\ does not silently use the wrong package manager.
• Update all nested pnpm call sites on the local dev path, plus \scripts/generate-worker-types.mjs\ (flagged in pnpm run-local cannot start on Windows: spawnSync pnpm ENOENT #19).
• Surface spawn failures in \generate-worker-types.mjs\ when
  esult.error\ is set (misleading wrangler error today).

Fixes #19

Approach

Follows the approach discussed in #19 / @snowyukitty's analysis: no shell, no .cmd\ shim, arguments stay intact (including checkout paths with spaces).

Test plan

• Windows 10 (10.0.26200), Node v24.16.0, pnpm 11.17.0, path \E:\Projects\cloudflare-os\ (no spaces): \pnpm run-local\ completes install/build and reaches wrangler ready on port 8787
• Checkout path containing a space (regression case from pnpm run-local cannot start on Windows: spawnSync pnpm ENOENT #19)
• \pnpm types:generate\ on Windows

CLA

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[Sainigurnoor511]

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