fix(core): break skill import cycle

[armancharan]

Issue for this PR

Closes #37424

#37424

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Importing @opencode-ai/core/skill first crashed on a cycle through permission and session. SkillNotFoundError was reading SkillV2.ID before that module finished loading.

Use Skill.ID from the schema package for the error and skill input type instead. Same ID schema, no init-order dependency.

How did you verify your code works?

• Reproduced the crash on v2, then confirmed the import exits 0 after the change
• bun test test/public-import.test.ts
• bun run typecheck in packages/core

Screenshots / recordings

n/a

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for your contribution!

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1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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