fix(opencode): align skills path across bootstrap, runtime, and tests

The bootstrap text advertised a configDir-based skills path that didn't
match the runtime path (resolved relative to the plugin file). Tests
used yet another hardcoded path and referenced a nonexistent lib/ dir.

- Remove misleading skills path from bootstrap text; the agent should
  use the native skill tool, not read files by path
- Fix test setup to create a consistent layout matching the plugin's
  ../../skills resolution
- Export SUPERPOWERS_SKILLS_DIR from setup.sh so tests use a single
  source of truth
- Add regression test that bootstrap doesn't advertise the old path
- Remove broken cp of nonexistent lib/ directory

Fixes #847

.opencode/plugins/superpowers.js

@@ -68,8 +68,6 @@ When skills reference tools you don't have, substitute OpenCode equivalents:
 - \`Skill\` tool → OpenCode's native \`skill\` tool
 - \`Read\`, \`Write\`, \`Edit\`, \`Bash\` → Your native tools
 
-**Skills location:**
-Superpowers skills are in \`${configDir}/skills/superpowers/\`
 Use OpenCode's native \`skill\` tool to list and load skills.`;
 
     return `<EXTREMELY_IMPORTANT>