Add GET / discovery endpoint for agent navigation

[PunchTheDev]

Summary

Root / was a 404. Agents arriving at the bare API host had no way to discover what was available without out-of-band knowledge.

GET / now returns:

• API name, version, description
• Docs links (/docs, /redoc, /openapi.json)
• Dashboard URL
• Labelled endpoint map (active rounds, specs, leaderboards, SOTA, submit, preview, health)
• One-line quickstart

This is the "agent persona" path from the working method — an agent that knows nothing must be able to land on the API and keep working.

Changes

• app/main.py: new @app.get("/") returning the discovery payload; version bumped to 0.15.7.
• tests/test_health.py: test_root_discovery asserts shape (name, version, docs, endpoints, quickstart). 138 → 139 tests.
• CHANGELOG.md: 0.15.7 entry.

Test plan

• pytest tests/test_health.py — 6 passed
• Full suite — 139 passed