[FEATURE]: stop using Effect.die for skill-not-found in the skill tool #40091

Feature hasn't been suggested before.

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Describe the enhancement you want to request

Per ERR-4 in packages/opencode/specs/effect/todo.md, clean up the
Effect.die(...) call in packages/opencode/src/tool/skill.ts. It
violates the rule in packages/opencode/specs/effect/guide.md that says:
Do not use Effect.die(...) for missing-resource failures.