[feature]: --json output for loopflow run #11

What problem would this solve?

The README sells LoopFlow as cron- and CI-friendly via exit codes, but there's no machine-readable output. CI pipelines that want to record cost or which step failed have to scrape the pretty console output.

Proposed solution

Add loopflow run --json that suppresses the pretty reporter and emits the RunResult (outcome, iterationsUsed, costUsd, per-step session ids) as a single JSON object to stdout. The RunResult type already exists in src/core/runner.ts — this is mostly a reporter/CLI flag change.

Alternatives considered

No response

Scope

• I've checked this isn't already on the roadmap in the README.
• This can't reasonably be expressed as a loop instead of engine code.