ci: preserve hidden Pages files

[steipete]

Summary

• preserve the generated .nojekyll file in the GitHub Pages artifact
• opt into hidden-file packaging required by actions/upload-pages-artifact@v5

Root cause

The v5 action defaults include-hidden-files to false. Sag's docs builder generates dist/docs-site/.nojekyll, so the recent action upgrade silently excluded that file.

Proof

• pnpm check
• pnpm build
• pnpm docs:build
• actionlint .github/workflows/*.yml
• local artifact emulation: default v5 exclusion omits .nojekyll; explicit opt-in includes it
• autoreview: clean, no accepted/actionable findings

Risk: low; one documented action input, limited to Pages artifact packaging.

[steipete]

Landed as 01b43135f786640e8d0e13dac6e80f62a4c9804c.

Proof:

• pnpm check
• pnpm build
• pnpm docs:build
• actionlint .github/workflows/*.yml
• local artifact emulation confirmed the v5 default omits .nojekyll and the explicit opt-in includes it
• autoreview clean: no accepted/actionable findings
• PR exact-head CI: https://github.com/steipete/sag/actions/runs/28505047971
• merged-main CI: https://github.com/steipete/sag/actions/runs/28505139447
• merged-main Pages deployment: https://github.com/steipete/sag/actions/runs/28505139409
• downloaded deployed github-pages artifact contains ./.nojekyll; artifact tar SHA-256 dd0e70cb2193329bedc99cfb8a5db99463952eb373c02080941500a3e8d8af7c

Live runtime proof: not applicable to this workflow-only packaging fix; the real Pages artifact and deployment are the affected boundary. No caveats.