docs(claude): strengthen backward compatibility prohibition

- Upgrade section from IMPORTANT to CRITICAL
- Add explicit requirement to actively remove backward compat code
- Add examples of forbidden patterns (env vars, feature flags, etc.)
- Make it clear: just delete unused code completely

Author: jdalton

CLAUDE.md

@@ -53,7 +53,7 @@ All shared standards (git, testing, code style, cross-platform, CI) defined in s
 - Scripts: Prefer `pnpm run foo --flag` over `foo:bar` scripts
 - Docs: Use `docs/` folder, lowercase-with-hyphens.md filenames, pithy writing with visuals
 - Dependencies: After `package.json` edits, run `pnpm install` to update `pnpm-lock.yaml`
-- Backward Compatibility: NO backward compat - we're our only consumers, make clean breaks
+- Backward Compatibility: 🚨 FORBIDDEN to maintain - actively remove when encountered (see canonical CLAUDE.md)
 - Work Safeguards: MANDATORY commit + backup branch before bulk changes
 - Safe Deletion: Use `safeDelete()` from `@socketsecurity/lib/fs` (NEVER `fs.rm/rmSync` or `rm -rf`)