#231 fix(ci): bump checkout/setup-node/cache actions to v5

Summary

• Bumps actions/checkout, actions/setup-node, and actions/cache from v4 to v5 across ci.yml and secret-scan.yml, clearing the post-merge deprecation annotations GitHub adds for these versions.
• Scoped to exactly what's flagged — actions/upload-artifact@v4 and denoland/setup-deno@v2 are untouched.

Context

Mined out of an old uncommitted codex/fix-ci-annotations branch that had drifted far behind main. That branch's other changes (dead-import cleanup in evidence-panels.tsx/ClinicalDashboard.tsx/answer-content.tsx, and a duplicate "Add generated sitemap audit" commit) turned out to be stale — the dead-code cleanup was valid for an old snapshot of those files but would delete functionality (EvidenceGapPanel, AnswerInsightBar, EvidenceVerificationStrip, etc.) that main has since gained, and the sitemap-audit work already merged separately via #224. This action-version bump is the one piece that's still valid and not yet on main.

Test plan

• Diffed against the original commit to confirm this reproduces exactly the same scoped change, re-applied cleanly on current main
• Full file read-through to confirm YAML structure/indentation is intact
• CI itself will confirm the workflow still runs correctly

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