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

The previous command referenced a non-existent script
(scripts/dev/validate_local.py). The correct entry point is
st-validate-local from standard-tooling.

Fixes #435

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

Author: wphillipmoore

docs/repository-standards.md

@@ -35,9 +35,7 @@
 
 ## Local validation
 
-- `uv run python3 scripts/dev/validate_local.py`
-- Docs-only changes: `uv run python3 scripts/dev/validate_docs.py`
-- Docs-only validation requires `markdownlint` on the PATH.
+- `st-validate-local`
 
 ## Linting policy