fix(cli): decode redirected Windows input as UTF-8

[rudycelekli]

Problem

Windows Python can decode redirected stdin with an ANSI code page, while upstream tools emit UTF-8 JSON. The CLI currently changes stdout/stderr to UTF-8 but leaves stdin unchanged. A Chinese XHS title therefore becomes mojibake through agent-reach format xhs, even though parsing succeeds.

Changes

• Reconfigure redirected Windows stdin to strict UTF-8 before reading it.
• Preserve interactive console input, text-only streams and the existing pytest capture guard.
• Report invalid UTF-8 as a clear format error with a failing exit status.

Verification

• Native child processes with raw UTF-8 stdin and PYTHONIOENCODING=cp1252, using a simulated Windows platform: unchanged main produces corrupted title/description.
• Focused regressions: unchanged main 2 failures and 3 controls; fixed branch 5 pass.
• Full test suite: 612 passed; Ruff and diff whitespace checks pass.
• Native Windows was not run; the code-page boundary is reproduced in real subprocesses on macOS with a simulated platform flag.
• MyPy retains unchanged-main MCP SDK v2 errors; this change adds none.

AI-assisted implementation and tests, independently reviewed before submission.