chore: update INVALID_DIRS to include plural 'skills' directory

[connorads]

Add skills/ to invalid OpenCode config directories list to prevent common plural naming mistake.

OpenCode uses singular directory names (skill/, agent/, command/, plugin/, tool/) for now.

Just in case people coming from other coding agents and miss the difference and get frustrated trying to get skills working. This will help them fail fast.

https://opencode.ai/docs/skills/

Testing

Before: no warning ❌

➜ opencode

After ✅

➜ bun dev 
$ bun run --cwd packages/opencode --conditions=browser src/index.ts
^[[I^[[I^[[IDirectory "skills" in /Users/connorads/git/opencode/packages/opencode/.opencode is not valid. Rename the directory to "skill" or remove it. This is a common typo.