feat(opencode): use native skills and fix agent reset bug (#226)

[obra]

Summary

• Replace custom use_skill/find_skills tools with OpenCode's native skill tool
• Use experimental.chat.system.transform hook instead of session.prompt (fixes OpenCode: bootstrap injection can reset custom agent to build (possible noReply interaction) #226 agent reset on first message)
• Symlink skills directory into ~/.config/opencode/skills/superpowers/
• Comprehensive Windows installation docs

Changes

Plugin (superpowers.js)

• Removed custom tools - rely on OpenCode's native skill discovery
• Replaced session.prompt({ noReply: true }) with experimental.chat.system.transform hook
• Simplified code, removed dependency on skills-core.js

Docs (README.opencode.md)

• Added skills symlink step to installation
• Complete Windows instructions for:
  • Command Prompt
  • PowerShell
  • Git Bash (using cmd //c mklink with cygpath)
• Reinstall safety with cleanup steps
• Verification commands for each shell
• WSL guidance

Test plan

• Verified plugin loads on OpenCode v1.1.32
• Verified bootstrap injected (model confirms "superpowers instructions")
• Verified agent doesn't reset (Plan stayed Plan after first message)
• Verified native skills discovered (superpowers/* visible)
• Windows testing needed

Fixes #226

[coderabbitai]

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