fix(harness-cline): use native `skills` tool instead of manual file-based workaround (#19880)

## Background

The Cline adapter runs in the host process, but harness skills were
materialized in the remote sandbox and advertised through the system
prompt instead of using Cline's native skill semantics.

## Summary

- Expose harness skills through Cline's native `skills` tool with
in-memory metadata and instruction loading.
- Include attached skill files in the native tool result and rebuild the
agent only when effective skill data changes.
- Remove sandbox skill writes and the extra readable-root exception
while retaining workspace confinement.
- Declare `skills` as a filterable, provider-executed Cline builtin and
add regression coverage.

## End-to-End Verification

Ran `./tools/run-harness-agent-examples.sh --harness cline --example
with-skills`.

## Checklist

- [x] All commits are signed with a verified signature.
- [x] Tests have been added or updated.
- [x] Documentation has been added or updated.
- [x] A patch changeset has been added.
- [x] The changes have been self-reviewed.