ci: overhaul workflows - lint, security scanning, release hardening

- add golangci-lint job to go.yml (parallel with build+test)
- add Go 1.23/1.24 version matrix, coverage only on 1.24
- upgrade setup-go@v4 to v5, codecov@v4 to v5 across all workflows
- fix check-large-files bug (find|while never exits 1), exclude .git/
- add concurrency groups to push+PR workflows (no duplicate runs)
- lowercase all workflow names to match project voice
- add gosec, errorlint, gocognit, nilnil, wastedassign, usetesting linters
- remove deprecated exportloopref (Go 1.22 fixed loop var capture)
- new: govulncheck.yml - Go vuln scanner with call-graph analysis
- new: scorecard.yml - OpenSSF supply chain scorecard
- new: dependabot.yml - auto-update Go deps + Actions versions
- release: SHA256 checksums + SBOM generation for all artifacts
- add CODEOWNERS

.github/dependabot.yml

@@ -0,0 +1,17 @@
+version: 2
+updates:
+  - package-ecosystem: gomod
+    directory: /
+    schedule:
+      interval: weekly
+    open-pull-requests-limit: 5
+    labels:
+      - deps
+
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly
+    open-pull-requests-limit: 5
+    labels:
+      - deps