Speed up install with --prefer-offline --no-audit --no-fund, fix postinstall path resolution

Install down from 60s to ~10s. Core packages only (11), heavy optional
packages available via feynman packages install.

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@companion-ai/feynman",
-  "version": "0.2.5",
+  "version": "0.2.6",
   "description": "Research-first CLI agent built on Pi and alphaXiv",
   "type": "module",
   "engines": {