fix: decode ZCode hook input as UTF-8 on Windows

[Grivn]

Windows PowerShell can decode ZCode's UTF-8 hook payload as CP936, corrupting Chinese text and losing stop_hook_active. The Stop hook then repeats its blocking reminder. Decode stdin explicitly as UTF-8, preserve the source BOM required for Chinese completion tokens, and accept the documented Chinese evaluations in both PowerShell and shell hooks. The reminder now requests a [mnemon] completion marker for other languages.

Closes #142.

Actual input	Original asset	Fixed asset
UTF-8 {"stop_hook_active":true,"last_assistant_message":"中"} with CP936 console input	Incorrect block	Silent exit
无需写入持久记忆 or 已写入记忆	Repeated block	Silent exit
Ordinary response without an evaluation	block	One reminder with completion-marker guidance

The opt-in runtime suite installs hooks with the built CLI in an isolated home containing spaces and Chinese characters, verifies the installed bytes, and runs both the original asset and the fixed hooks. Windows runs require powershell.exe 5.1. The regular deterministic CI gate remains unchanged; the Manual Integration workflow adds a targeted zcode-windows selection.

Validation:

• go build -o mnemon . and make test pass.
• Real CLI smoke, setup/reinstall/eject, and Memory CLI E2E pass (258/258 checks).
• Portable PowerShell 7 regression tests pass, including the original-asset controls. A separate three-code-page check improves from 19/27 to 27/27 expected outcomes.
• Native Windows PowerShell 5.1 run passes at this commit: 34 cases (30 fixed-hook cases and four original-asset controls), with console input code pages 65001 and 936. The runner's system ANSI code page is 1252; CP936 is selected explicitly at the input boundary.

After updating Mnemon, rerun mnemon setup --target zcode --global --yes and start a new ZCode session to refresh installed hooks. The verification covers actual installed hook processes; it does not exercise the ZCode GUI.