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>

.feynman/settings.json

@@ -6,13 +6,10 @@
     "npm:pi-web-access",
     "npm:pi-markdown-preview",
     "npm:@walterra/pi-charts",
-    "npm:pi-generative-ui",
     "npm:pi-mermaid",
     "npm:@aliou/pi-processes",
     "npm:pi-zotero",
-    "npm:@kaiserlich-dev/pi-session-search",
     "npm:pi-schedule-prompt",
-    "npm:@samfp/pi-memory",
     "npm:@tmustier/pi-ralph-wiggum"
   ],
   "quietStartup": true,