docs: rewrite README with setup, compaction config, and log verification

- Simplified quick start (4 steps)
- Added compaction section: how it works, threshold config, log verification
- Updated env var table (added MORPH_COMPACT_TOKEN_LIMIT, removed stale vars)
- Removed leftover debug comment artifacts from index.ts
- Updated test assertions for new README content

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>