docs: add engraph v2.2a design spec — context engine

Six context functions: read, list, vault_map, who, project, context.
Pure library functions + CLI subcommands. No server, no embedding
(except context topic which reuses search_internal).

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

Author: devwhodevs