Fix PowerShell Unicode path handling

[pradigmaz]

Summary

• write Windows PowerShell temp scripts with a UTF-8 BOM
• set PowerShell console/output encoding to UTF-8 before user code runs
• add Windows coverage for non-ASCII stdout and cwd handling

Verification

• npx vitest run tests/core/executor.test.ts
• npm run typecheck
• npm run build
• direct build/executor.js matrix: PowerShell 5 + pwsh7, ASCII + Japanese + Korean + Arabic + Hebrew + Greek + Hindi + Thai + accent + emoji paths

[pradigmaz]

Context against #796: this PR is intentionally narrower.

It only fixes PowerShell script/output encoding for non-ASCII paths/stdout:

• UTF-8 BOM for .ps1 temp scripts so Windows PowerShell reads them correctly
• UTF-8 console/output prelude for PowerShell/pwsh
• regression coverage for non-ASCII stdout + cwd

It does not change shell runtime selection, routing docs, or guidance text. Local Windows verification covered PowerShell 5 + pwsh7 across ASCII, Japanese, Korean, Arabic, Hebrew, Greek, Hindi, Thai, accented, and emoji path prefixes.