Add SpiderBrain understanding layer

[spiderbrainai]

SpiderBrain gives this repo a committed, source-free understanding layer in .spiderbrain/ plus an AGENTS.md block, so any AI coding agent that later touches the repo can read its structure, dependencies, and blast radius with zero setup:

npx spiderbrain mcp        # MCP server for Claude Code / Cursor
npx spiderbrain keystones  # the load-bearing files

Brain health (computed deterministically from the committed brain)

Signal	Read	Detail
🟢 Coverage	good	53 of 53 files mapped
🟢 Keystones	good	top file reaches 9 of 53 files
⚪ Blindspots	—	needs cloud scores
🔴 Orphans	issue	22 isolated files
⚪ Freshness	—	commit not recorded

The folder is source-free (paths, structure, edge-derivable blast only; no source, no scoring weights) and deterministic. Format: https://github.com/aabhisrv/spiderbrain.ai/blob/main/SPEC.md