fix(opencode): support both OpenCode V1 and V2 (#1557)

[ucirello]

Summary

The same plugin declaration now supports OpenCode V1 and V2. In OpenCode v2, slash commands attach the selected skill natively and pass command arguments as prompt text, instead of asking the model to issue a separate Load and execute... request.

Fixes #1557: Expand support for both OpenCode v1 and v2.

Design Decisions

• The top-level JavaScript entrypoint stays directly under .opencode/plugins/, so OpenCode v1 discovers one plugin while the v1 and v2 implementations remain separate helper modules.
• The v1 config hook adds the bundled skills path and creates slash-command wrappers.
• The v2 setup registers skill definitions through the current draft.add API.
• A v2 command such as /ce-plan add OAuth login sends text: "add OAuth login" and skills: [{ id: "ce-plan" }]. Existing prompt attachments are preserved and the selected skill is not duplicated.
• Skill frontmatter is removed before registration while the skill's real location is retained, so OpenCode can resolve supporting files relative to the skill.
• Skills marked non-user-invocable do not receive slash commands.

New concepts

Native v2 skill attachments

OpenCode v2 can attach a skill by ID when submitting a prompt. OpenCode then loads the skill content and supporting files from the registered skill location. This keeps the user request separate from the skill instructions and avoids a model-mediated skill-loading turn. Use this pattern when a command is a thin entry point to a registered skill. It is not a copy of the skill body into a command template.

Validation

• bun test tests/opencode-plugin-commands.test.ts - 8 passed.
• bun run release:validate - passed.
• bun run plugin:validate - passed.
• git diff --check - passed.
• OpenCode v1 1.18.23 - interactive TUI loaded the repository plugin and reached a clean exit (init count=38).
• OpenCode v2 0.0.0-beta-18314 - standalone startup loaded the repository plugin and reached the interactive TUI; native skill-command behavior was manually verified.
• bun run test - not completed within 420 seconds; it remained in tests/skills/ce-work-cross-model-routes.test.ts.
• bun run typecheck and bun run lint - not configured in this repository.

Security Disclosure

The plugin reads bundled SKILL.md files and passes their registered locations and content to OpenCode. No new shell execution, credential handling, permission rules, or dependencies were introduced.

Agent Disclosure

• Model: OpenCode · openai/gpt-5.6-luna

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