Task: add a /health endpoint

An example task for a full multiagent run:

niki run "Add a GET /health endpoint returning { status: 'ok', uptime }" --project ./my-app

What happens per stage:

• Planner: reads the repo, emits a TaskSpec: which file to touch, the approach.
• Coder: emits a unified diff against the active execution workspace.
• Tester: generates a test report and runs a test command when one is available.
• Reviewer: scores the evidence and can request another Coder pass within the configured revision limit.

In pipeline.topology = "auto", a low-complexity task can collapse to Planner plus a solo Coder.

When branch creation succeeds, output includes a niki/<id> branch, changes.patch, report.md and per-agent JSON artifacts under .niki/tasks/<id>/. Those artifacts are excluded from the published diff. safety_proof.json, when written, covers git invariants only and is skipped on an empty diff. The final diff is applied to your working tree for review.