Add Cactus agent plugin with evals

[rforgeon]

Summary

• Adds an installable Cactus agent plugin for runtime setup, mobile integration planning, voice-agent prototyping, and latency/privacy checks.
• Includes Codex and Claude plugin manifests where this PR adds a new plugin directory.
• Adds plugin eval cases that test the plugin behavior and expected output shape, not the host product itself.
• Documents a metadata-only telemetry boundary so plugin usage can be measured without capturing prompts, files, credentials, connector payloads, or model outputs.

Files

• plugins/cactus-agent-builder

Validation

• Validated Codex plugin manifests with the local plugin validator where this PR adds a new plugin.
• Validated eval JSONL with jq -c ..