fix(executor): handle Windows shell runtimes robustly

[NgoQuocViet2001]

What / Why / How

While dogfooding context-mode from Codex on Windows, ctx_execute(language: "shell") and ctx_batch_execute can fail before user code runs:

File ...\script.ps1 cannot be loaded because running scripts is disabled on this system.
UnauthorizedAccess

Codex sets SHELL to Windows PowerShell in this environment. context-mode correctly writes a temporary script.ps1, but then runs it as powershell -File <script>. On normal Windows PowerShell installs where the effective execution policy is Restricted, that temp script is blocked.

This patch keeps the fix narrow to Windows shell handling:

• run Windows PowerShell / pwsh temp scripts with -NoProfile -ExecutionPolicy Bypass -File
• use .cmd temp scripts plus cmd.exe /d /s /c when the selected shell is cmd
• ignore Windows WSL bash shims from SHELL overrides, so Windows-hosted execution falls back to Git Bash instead of C:\Windows\System32\bash.exe
• keep Git Bash, cmd, PowerShell 5, pwsh 7, and Unix shell command shapes covered by tests

ExecutionPolicy Bypass here is process-scoped for this spawned PowerShell process only; it does not change machine/user policy.

Validation

• pnpm exec vitest run tests/runtime.test.ts -t "SHELL env var|Windows bash gets bash -c source pattern|Windows powershell gets process-scoped execution policy bypass|Windows pwsh gets process-scoped execution policy bypass|Windows cmd gets cmd /c pattern|Unix bash gets direct file path" --reporter=dot
• pnpm exec vitest run tests/executor.test.ts -t "PowerShell shell runtime executes generated script|buildCommand returns shell command array|buildScriptFilename: POSIX shell on Windows has NO extension|buildScriptFilename: cmd on Windows uses .cmd extension|buildScriptFilename: PowerShell on Windows uses .ps1 extension|buildScriptFilename: shell on Unix keeps .sh extension" --reporter=dot
• Manual PolyglotExecutor smoke on this Windows machine: Windows PowerShell 5, pwsh 7, Git Bash, cmd all executed successfully.
• Manual MCP smoke against rebuilt server.bundle.mjs: ctx_execute and ctx_batch_execute succeeded for Windows PowerShell 5, pwsh 7, Git Bash, cmd, and WSL-bash-shim fallback.
• pnpm run typecheck
• pnpm run build