fix: use cross-spawn for Windows compatibility

[sawyer0x110]

Summary

• rebase the branch onto current main after test: harden adapter coverage across CLI and platform edge cases #69 so it only changes the agent spawn path
• switch ACP agent startup in src/client.ts from node:child_process.spawn() to cross-spawn while preserving the current buildAgentSpawnOptions() path and windowsHide behavior
• regenerate pnpm-lock.yaml and drop the stale npm lockfile from the old diff

Why this is still needed

Windows CreateProcess does not resolve .cmd/.bat launchers like npx.cmd through PATHEXT, so agent startup can still fail with spawn EINVAL even after the windowsHide work in #69. This PR keeps the Windows compatibility fix without reintroducing stale branch state.

Testing

• pnpm run typecheck:tsc
• pnpm run lint
• pnpm run build
• pnpm run test

Notes

• pnpm run format:check currently reports an existing formatting issue in CONTRIBUTING.md on current main; this branch does not modify that file.

Fixes #48

[vincentkoc]

Rebased onto current main and reduced the diff to the merge-relevant Windows fix only.

What changed:

• preserved the current buildAgentSpawnOptions() + windowsHide path from main
• switched only the agent spawn call in src/client.ts to cross-spawn
• regenerated pnpm-lock.yaml
• removed the stale package-lock.json that came from the original branch

Validation on the rebased branch:

• pnpm run typecheck:tsc
• pnpm run lint
• pnpm run build
• pnpm run test

pnpm run format:check still flags the existing CONTRIBUTING.md formatting issue from current main, which is outside this branch.