fix: opencode skills dir plural

[ethanfann]

Note

Corrects the OpenCode agent directory names in defaults to use plural skills.

• In config-store.ts, updates AGENT_DEFINITIONS for opencode from ~/.config/opencode/skill and .opencode/skill to ~/.config/opencode/skills and .opencode/skills
• Affects only default path resolution for OpenCode in DEFAULT_AGENTS; no behavioral or API changes elsewhere.

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