ci: reduce artifact storage retention

[BenSheridanEdwards]

Why

Reduce GitHub Actions artifact storage pressure so diagnostic uploads do not consume quota or mask real gate results.

What Changed

• Upload coverage and Playwright reports only on failure.
• Make diagnostic artifact uploads non-blocking.
• Retain coverage, Playwright, and dist artifacts for 3 days.

Proof

• pnpm verify: not run, workflow upload metadata only.
• pnpm e2e: not run, behaviour map unchanged.
• Visual proof attached or linked, or N/A with reason: N/A, no UI-visible change.
• StyleProof reviewed, or N/A with reason: N/A, no CSS/rendered surface change.
• Other proof: ruby -e 'ARGV.each { |f| require "yaml"; YAML.load_file(f); puts "yaml ok #{f}" }' .github/workflows/ci.yml; actionlint .github/workflows/ci.yml; git diff --check all passed.

CI Status

• Latest-sha GitHub Actions are green: Analyze, Fallow, CodeQL, visual gate, E2E, Lighthouse, build, audit, typecheck/lint/format, unit/component, and StyleProof passed on 5da3033.
• Pending checks are named below: none.
• Failing checks are named below with the fix plan: none.

Risk And Rollback

Low risk. Successful runs will keep less diagnostic output. Failed runs still keep coverage/Playwright diagnostics for 3 days. Roll back this workflow file if longer retention is needed.

Definition Of Done

• Behaviour is covered by the E2E map when user-visible behaviour changed: N/A, no behaviour changed.
• Logic is unit-tested where logic changed: N/A, no logic changed.
• Accessibility was considered and verified for interactive/UI changes: N/A, no UI changed.
• The diff follows .agents/project/ARCHITECTURE.md and CONVENTIONS.md for a workflow-only change.
• Any new architectural decision is recorded as an ADR: N/A, no architecture decision.
• No gates were bypassed or weakened.

[github-actions]

Fallow audit report

Found 1 finding.

Details

Severity	Rule	Location	Description
minor	fallow/unused-dev-dependency	package.json:44	Package '@lhci/cli' is in devDependencies but never imported

Generated by fallow.

[github-actions]

Fallow audit report

0 inline findings selected for GitHub review.

[github-actions]

🗺️ StyleProof report

✓ No visual changes detected.