Fix Windows shell runtime and PowerShell UTF-8 handling

[Niche-alchemy]

Summary

• Prefer pwsh over Windows PowerShell 5.1 when Git Bash/POSIX shell is unavailable.
• Prefix PowerShell shell execution with UTF-8 input/output encoding setup.
• Update Codex/Claude guidance to recommend ctx_execute(language: "javascript") for portable filtering/counting instead of bash-only shell
  examples.
• Add regression tests for Windows shell selection and PowerShell UTF-8 script prolog.

Why

On Windows, ctx_execute(language: "shell") may run under PowerShell instead of POSIX shell. Bash-style examples such as grep/head/tail can silently fail or produce no useful output. PowerShell 5.1 also defaults to non-UTF-8 encodings, which breaks Chinese paths/output.

Verification

• npm run typecheck
• vitest run tests/runtime.test.ts -t "Windows prefers pwsh over powershell when bash unavailable"
• vitest run tests/executor.test.ts -t "buildPowerShellScriptContent prefixes UTF-8 encoding setup"
• npm run bundle
• npm run assert-bundle

[ken-jo]

@Niche-alchemy Thanks for the PR.

I validated this following the maintainer review process, including git archaeology, targeted runtime tests, and native Windows checks.

What this PR is trying to fix

This PR aims to improve Windows shell runtime behavior and PowerShell UTF-8 handling:

• PowerShell / pwsh shell execution gets UTF-8 input/output setup.
• Windows shell detection chooses an available shell in a defined order.
• The routing/docs guidance moves away from host-shell-specific language: "shell" examples where a language-neutral ctx_execute example is safer.

Validation performed

Linux:

• npm run build --silent: passed
• npx vitest run tests/runtime.test.ts tests/executor.test.ts --reporter=dot: passed, 143 passed / 28 skipped

macOS:

• npm run build --silent: passed
• Targeted PowerShell/runtime tests passed

Windows:

• npm run build --silent: passed
• tests/executor.test.ts -t buildPowerShellScriptContent: passed
• tests/runtime.test.ts -t "Windows prefers pwsh" failed on a native Windows machine where Git Bash is installed

Failure:

expected 'C:\Program Files\Git\usr\bin\bash.exe' to be 'pwsh'

I also ran a direct PowerShell UTF-8 e2e through PolyglotExecutor. The process exited with code 0, but stdout was mojibake instead of the expected non-ASCII string:

powershell {"exitCode":0,"stdout":"?.... mojibake output ....\r\n","stderr":""}

Interpretation

The patch direction is useful, but the current Windows behavior does not prove the core UTF-8 claim yet. The new runtime test also fails in a realistic Windows environment where Git Bash is installed.

Requested changes

Could you please update this PR with the following?

1. Clarify and fix the Windows shell-selection expectation when Git Bash is installed.
2. Add or update a native Windows e2e test proving that PowerShell/pwsh returns non-ASCII stdout correctly through PolyglotExecutor.
3. Re-check whether the docs/routing example in src/server.ts should use execSync, because it throws on failing commands before the filtering logic can run.

Once those are updated, this should be re-tested on Windows specifically.