fix: update validation command references to st-validate-local

Also fixes the PR template which referenced the same non-existent script.

Fixes #435

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: wphillipmoore

.github/pull_request_template.md

@@ -13,8 +13,7 @@
 
 ## Testing
 
-- markdownlint
-- uv run python3 scripts/dev/validate_local.py
+- `st-validate-local`
 
 ## Notes