🚀 Fork Sync Pack v5.0.9: production fixes + OpenCode integration (ready for selective cherry-pick)

[GofMan5]

Hi maintainers,

I prepared a fork-side integration pack that is already running in production on v5.0.9, and may be useful for selective upstream cherry-pick.

Fork repo (reference build):

• https://github.com/GofMan5/Antigravity-Manager

What is included (high impact parts):

• Proxy stability/security hardening (headless startup persistence, auth boundary handling, path safety checks).
• Thinking/signature isolation by session for OpenAI/Claude/Gemini mapper flows.
• OpenCode sync integration end-to-end:
  • Rust backend module + tauri commands
  • admin HTTP routes (/api/proxy/opencode/*)
  • web invoke mapping
  • CLI Sync card integration in UI
  • i18n keys synced across locales

Related PR for code review context:

• feat: sync fork improvements including proxy hardening and OpenCode integration #1668

If this direction looks useful, I can split it into smaller focused patches (proxy hardening / mapper signatures / opencode integration) for easier review.