feat: allow /eval agent to load external fixture files

[sdhilip200]

Summary

• Reuse the external fixture resolver in the interactive /eval agent flow so it accepts built-in fixture IDs and fixture JSON paths.
• Add regression coverage for resolving an external fixture path from /eval agent.
• Document the interactive fixture-path form in README and Quickstart.

Verification

• cargo test
• cargo clippy --all-targets -- -D warnings
• cargo fmt --all -- --check
• git diff --check

Closes part of #12.