ci: adopt standard-actions v1.5 reusable workflows and remove st-validate-local

[wphillipmoore]

Summary

Adopt standard-actions v1.5 reusable CI workflows and remove legacy
st-validate-local scripts.

Changes

1. Replace bespoke ci.yml with v1.5 reusable workflows (ci-quality,
   ci-security, ci-release)
2. Delete legacy validation scripts (lint.sh, test.sh, audit.sh)
   replaced by st-validate
3. Update documentation references from st-validate-local to
   st-docker-run -- st-validate
4. Run st-github-config apply to enforce repo settings

[wphillipmoore]

Closed after finalization. PR: #184

[wphillipmoore]

Reopened — PR #184 is still open (CI green). Issue was closed prematurely.

[wphillipmoore]

Closed after finalization. PR: #184