feat: add OpenCode native plugin support

[myphsto]

Summary

Add native OpenCode plugin support via .opencode/plugins/trailofbits-skills.js which automatically discovers skills from plugins/*/skills/ directories.

Changes

• .opencode/plugins/trailofbits-skills.js — ES module plugin that auto-discovers all plugins/*/skills/ directories and injects them into OpenCode's config
• package.json — root-level {"type": "module"} required by Node.js v26 for ES module support
• .opencode/.gitignore — ignores node_modules/
• .github/scripts/check_opencode_loadability.py — end-to-end CI validation: runs opencode debug skill, verifies all skills load with valid frontmatter
• .github/workflows/validate.yml — installs OpenCode CLI, runs loadability check
• README.md — adds OpenCode installation section
• AGENTS.md — updates cross-platform compatibility section, adds OpenCode to PR checklist

Testing

• All 74 Trail of Bits skills load through OpenCode
• Python script passes ruff check + format
• End-to-end opencode debug skill validation passes
• All existing Claude Code and Codex loadability checks continue to pass

Notes

• This is purely additive — no changes to existing Claude Code or Codex behavior
• The .opencode/ directory is inert for all other platforms
• OpenCode auto-discovers skills, so no per-plugin metadata is needed

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[CLAassistant]

All committers have signed the CLA.

[myphsto]

Added this on my fork figured if anyone is interested.