ci(turbo): pass PATHEXT and ComSpec through on Windows so the pnpm shim finds pnpm.exe

[BSalaeddin]

Turbo runs tasks under strict env mode by default, which strips every variable a task does not declare. On Windows that includes PATHEXT and ComSpec, and stripping PATHEXT breaks every task turbo runs here: the pnpm on PATH is a shim that re-executes the engine pinned by packageManager, and without PATHEXT it looks for an extensionless pnpm, never considers pnpm.exe, and exits with ERR_PNPM_ENGINE_BIN_MISSING. Upstream fixed this in vercel/turborepo#13114 by adding both variables to turbo's default passthrough list, shipped in turbo v2.10.0. This repo is on turbo 2.5.4, so the passthrough is declared in turbo.json instead.

Proof on the affected machine, in a clean worktree of this branch, with no --env-mode=loose:

# before (this commit reverted)
pnpm exec turbo run lint --force
 Tasks:    0 successful, 2 total
 Failed:   caramel-extension#lint

# after
pnpm exec turbo run lint --force
 Tasks:    2 successful, 2 total

This adds globalPassThroughEnv only and declares no build-time variables, so the Dockerfile rule documented in CLAUDE.md is untouched: the image build stays pnpm --filter caramel-app run build, never turbo run build.

No-op on Linux CI: neither variable exists on the runners, and passing through an unset variable changes nothing.

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