fix(harness-acp): place skills in their harnesses' natively supported directory instead of applying a manual workaround (#19751)

## Background

ACP implementations already support native skill discovery, but the
generic adapter wrote skills to a private session directory and manually
advertised them through the prompt.

This was likely chosen because ACP itself does not natively support
providing skills. But that still shouldn't have led to the workaround.
Since skills are supported natively by basically every single harness,
the ACP meta harness adapter should simply place the files in the
correct directory per concrete ACP backed adapter.

## Summary

- Write skills to the implementation's native directory under its
effective `$HOME`, defaulting to `.agents/skills`.
- Add `skillsDirectory` for runtime-specific overrides, e.g. for Claude
ACP to use `.claude/skills`.
- Remove the manual prompt catalog while preserving resume-safe skill
materialization.

## End-to-End Verification

Ran the `with-skills` harness example successfully for all ACP backed
harness adapters.

## Checklist

- [x] All commits are signed (PRs with unsigned commits cannot be
merged)
- [x] Tests have been added / updated (for bug fixes / features)
- [x] Documentation has been added / updated (for bug fixes / features)
- [x] A _patch_ changeset for relevant packages has been added (for bug
fixes / features - run `pnpm changeset` in the project root)
- [x] I have reviewed this pull request (self-review)