fix: make harness example self-contained

[ForeverAngry]

Summary

• vendor the example-only harness record helper into rig-mcp/examples/common
• repoint the harness example at the local helper so isolated CI checkouts do not need a sibling rig-compose repo

Validation

• cargo fmt --all -- --check
• just check

[Copilot]

Pull request overview

This PR makes the harness_record example self-contained by vendoring the shared “harness record” helper into this repository’s examples/ tree, removing the need for a sibling rig-compose checkout in isolated CI environments.

Changes:

• Repoint examples/harness_record.rs to use a local examples/common/harness_record.rs module via #[path = ...].
• Add a new examples/common/harness_record.rs with serde-serializable mirror types and a helper constructor for converting from rig-compose invocation/result types.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
examples/harness_record.rs	Updates module path to the vendored example helper to avoid external relative paths.
examples/common/harness_record.rs	Adds the shared example-only harness record schema/types and conversion helper.

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