docs: add engraph v2.1 implementation plan — graph & graph agent

8-task TDD plan: edges table, graph helpers, wikilink extraction,
graph agent expansion, indexer edge building, 3-lane RRF integration,
graph CLI, version bump.

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

Author: devwhodevs