Add OpenCode installation support

[sgraczyk]

Summary

Provide an official OpenCode installation path similar to Superpowers, so users can install spec-first directly in OpenCode.

Proposal

• Add .opencode/INSTALL.md with OpenCode-specific instructions.
• Implement an OpenCode plugin entry point (JS/TS) and document it in README.
• Allow installation via opencode.json plugin entry, e.g.:

{
  "plugin": ["spec-first@git+https://github.com/bitcraft-apps/spec-first.git"]
}

• Ensure skills are discoverable under OpenCode (e.g. .opencode/skills/ or Claude-compatible skill paths).

Notes

OpenCode supports plugins via opencode.json and auto-installs them at startup. Skills are Claude-compatible, but plugins are not.

Acceptance criteria

• Users can install spec-first in OpenCode following documented steps.
• OpenCode loads spec-first plugin and skills without manual symlinks.