perf: speed up skill directory discovery

[thdxr]

Summary

• split skill initialization into a cheap discovery pass and a full parse pass so Skill.dirs() no longer loads every skill file
• keep full skill parsing for get, all, and available so skill content and validation behavior stay unchanged
• reduce startup work in code paths that only need skill directories for permissions and whitelisting

Verification

• bun typecheck in packages/opencode from the main checkout
• bun test test/skill/skill.test.ts test/tool/skill.test.ts in packages/opencode from the main checkout

[greptile-apps]

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