fix setup support for opencode host

[monaChoy]

Summary

• fix ./setup --host opencode, which is documented in README.md but currently rejected by the shell installer
• add OpenCode handling to setup, including host validation, auto-detection, skill doc generation, and skill installation
• verify the OpenCode install flow end to end by running ./setup --host opencode

Problem

README.md documents OpenCode support with:

• ./setup --host opencode
• install path: ~/.config/opencode/skills/gstack-*

However, the current setup script does not accept opencode as a valid host, so the documented install path is broken.

The repository already includes an OpenCode host definition in hosts/opencode.ts; this change wires that documented host into the existing shell installer flow.

Testing

• installed bun
• ran ./setup --host opencode
• confirmed successful output: gstack ready (opencode).
• confirmed generated skills under ~/.config/opencode/skills
• confirmed runtime root under ~/.config/opencode/skills/gstack

[monaChoy]

Closing this in favor of #982, which addresses the same opencode setup gap with broader coverage and test updates.