feat: add support for OpenCode

[ryzizub]

Description

OpenCode (https://opencode.ai). Tier-2 / wave-1 target under epic #66, alongside Codex (#226) and Gemini CLI (#227).

OpenCode is a "universal" agent: it reads .agents/skills/ directly (also .opencode/skills, ~/.config/opencode/skills, and .claude/skills), so once the repo is published to the open standard (#224), the workflow skills land in OpenCode for free.

Scope

• Skills — verified to load from .agents/skills/.
• Hook — port recommend-plugins. NOTE: OpenCode uses a JS/TS plugin API, not the Claude JSON-over-stdin shell-hook contract, so this is a rewrite as an OpenCode plugin (more effort than Codex/Gemini, which mirror Claude's contract).
• MCP — wire context7 via opencode.json.
• Subagents — OpenCode supports markdown subagents (mode: subagent, invoked via @mention), close to our existing agents/*.md. Port the review/plan/research panels as OpenCode agents.
• AGENTS.md — OpenCode reads AGENTS.md natively (see feat: support AGENTS standard #225).
• CI — loader job asserting skills load in OpenCode.

Docs: https://opencode.ai/docs/agents/ , https://open-code.ai/en/docs/skills

Requirements

• Skills load in OpenCode via .agents/skills/
• recommend-plugins hook ported as OpenCode plugin
• context7 MCP wired via opencode.json
• review/plan/research agents ported as OpenCode subagents
• OpenCode loader CI job added

Additional Context

Part of epic #66. Tier 2 / wave 1 alongside Codex (#226) and Gemini CLI (#227).