fix: enable shell for terminal spawn on Windows

[Bortlesboat]

Summary

• Add shell: process.platform === "win32" to buildTerminalSpawnOptions() so terminal commands resolve from PATH on Windows
• On Windows, child_process.spawn() without shell: true doesn't use cmd.exe for PATH resolution, causing every terminal command to fail with ENOENT
• Linux/macOS handle this in the kernel during execve(), so shell is only enabled on Windows

Test plan

• Added test in spawn-options.test.ts verifying shell is always a boolean
• All existing spawn-options tests pass
• oxlint, oxfmt, and tsc --noEmit clean
• Verified on Windows 11 — git, node, and other PATH commands now resolve correctly

Closes #169

[osolmaz]

Triage result

Human attention: ⚠️ Required
Recommendation: 🏁 escalate to a human
Human decision needed: ready for human landing decision

Quick read

This PR addresses a real Windows bug in ACP terminal spawning. The targeted regression was reproduced and shown fixed, the branch is conflict-clean against origin/main, and CI is green for the current head 0635be71852d1ea0b44697908981f4cfc2e4367b.

Intent

Make ACP terminal commands work on Windows so commands like git, node, and other PATH-resolved executables run normally instead of failing with ENOENT.

Why

The underlying problem is that buildTerminalSpawnOptions() did not enable the Windows shell path needed for PATH/PATHEXT command resolution. That caused Windows terminal executions to fail even when the command existed in PATH.

Codex review

GitHub Codex review data for the current head is empty. The established local Codex review for the refreshed head reports no remaining P0/P1 issues. There is one non-blocking P2 note about case-insensitive PATH/PATHEXT override handling during terminal spawn detection, but it did not rise to a blocking safety issue for continuing.

CI/CD

The current CI workflow run 23673820556 completed successfully, and gh pr checks shows all required jobs passing: scope, Format, Typecheck, Lint, Build, Conformance Smoke, Test, and Docs. An older action_required CI run exists for superseded head 1294310ac8980e2faf212c890458deafb9fecfd9, but it is unrelated to the current branch state. Final conflict check is clean.

Recommendation

Recommendation: 🏁 escalate to a human. This is ready for human landing decision. If the remaining non-blocking P2 is acceptable to defer, the PR is in a good state to land from the current head.

[osolmaz]

Landed via temp rebase onto main.\n\n- Gate: validation completed for this repo's change scope\n- Land commit: d0525df\n- Merge commit: 6e0b939\n\nThanks @Bortlesboat!