ci: slim down push-to-main CI and remove redundancies

[arimxyer]

Summary

• Skip integration tests on push to main — PRs already run the full 3-OS matrix; push to main now only runs lint + unit tests + build (fast sanity check for direct pushes)
• Remove security job from CI — the weekly security-scan.yml workflow covers gosec scanning; per-PR scanning was redundant
• Delete pr-validation.yml — was a no-op that only echoed PR metadata

Rulesets updated via API (already applied):

• Main branch: removed "Security Scan" from required checks
• Release tags: reduced to Lint + Unit Tests + Build (matches what push-to-main provides, so tag creation works without admin bypass)

CI flow after this change

Event	Jobs that run
pull_request	Lint, Unit Tests, Integration Tests (3 OS), Build
push to main	Lint, Unit Tests, Build
Tag v*	Release workflow (separate)
Weekly cron	Security scan (separate)

Test plan

• PR CI runs full suite (this PR itself validates it)
• After merge, push-to-main CI skips integration tests
• workflow_dispatch still works for manual full runs

🤖 Generated with Claude Code