feat: add ZCode memory integration

[Grivn]

Summary

• add mnemon setup --target zcode detection, install, and eject support
• install a native Mnemon SKILL.md at project or user scope
• register user-level SessionStart, UserPromptSubmit, and Stop process hooks in ~/.zcode/cli/config.json when --global is selected
• preserve unrelated ZCode skills, hook events, and settings during repeated setup and eject
• document the integration in English and Chinese

Why

ZCode exposes a native skill format and a user-level hook protocol, so Mnemon can participate in the host lifecycle without adding another service. ZCode currently ignores project-level hook configuration; the setup flow therefore installs only the project skill locally and reserves lifecycle hooks for --global installs.

Upstream contracts used for this adapter:

• https://zcode.z.ai/en/docs/skill
• https://zcode.z.ai/en/docs/hooks

User impact

Users can run mnemon setup --target zcode --global --yes, restart ZCode, and receive session priming, per-prompt recall guidance, and bounded stop-time writeback guidance. Eject removes only Mnemon-owned entries.

Validation

• make test
• go build -o /tmp/mnemon-zcode-build .
• temporary-HOME setup, hook-protocol, and eject smoke test