docs: add Platform-agnostic design principle to CLAUDE.md

Codifies the "Learn Your Project" rule: skills must never hardcode
framework-specific commands. Read CLAUDE.md, ask if missing, persist
the answer. Prevents Rails-isms from recurring.

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

CLAUDE.md

@@ -94,6 +94,18 @@ Rules:
 - **Express conditionals as English.** Instead of nested `if/elif/else` in bash,
   write numbered decision steps: "1. If X, do Y. 2. Otherwise, do Z."
 
+## Platform-agnostic design
+
+Skills must NEVER hardcode framework-specific commands, file patterns, or directory
+structures. Instead:
+
+1. **Read CLAUDE.md** for project-specific config (test commands, eval commands, etc.)
+2. **If missing, AskUserQuestion** — let the user tell you or let gstack search the repo
+3. **Persist the answer to CLAUDE.md** so we never have to ask again
+
+This applies to test commands, eval commands, deploy commands, and any other
+project-specific behavior. The project owns its config; gstack reads it.
+
 ## Browser interaction
 
 When you need to interact with a browser (QA, dogfooding, cookie setup), use the