Add machine-readable JSON output for Emberfall test results

[talentedmrjones]

Summary

Emberfall currently reports test results as human-readable stdout and does not expose an output format flag such as --output json (cmd/root.go, README.md). Per-test reporting is printed from test.report(), and the final Ran / Failed / Skipped summary is printed at the end of engine.Run. That makes the output easy for a person to read, but hard for CI or other tools to consume without scraping free-form text.

Requested change

• Add a machine-readable JSON output mode for Emberfall test runs.
• Include per-test result data and final summary counts in that output.
• Keep the existing human-readable output available.
• Keep the GitHub Action aligned with the same behavior, since it shells out to the CLI today (action.yml).

Acceptance criteria

• The CLI provides a structured JSON output mode.
• JSON output includes per-test status and failure details, plus overall run summary counts.
• Existing human-readable output remains available.
• Docs and CLI tests are updated to cover the new behavior.