docs: add engraph v2.1 design spec — graph & graph agent

Vault graph with wikilink + mention edges, graph search agent as
3rd RRF lane, engraph graph CLI subcommand. Context engine deferred
to v2.2 where it ships alongside MCP server.

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

Author: devwhodevs