Update docs for slide and CI workflows

[cursor]

Docs added/updated

• Updated README and S4R agent docs to reflect the current 26-slide Marp presenter deck.
• Expanded demo-slides README with operational constraints for local-only Claude Enterprise business-case Marp targets.
• Updated CI/CD and developer docs to describe gitleaks coverage, unfiltered PR CI trigger behavior, and Marp/local-only documentation ownership.

Codepaths covered

• Makefile Marp targets: marp-preview, marp-serve, marp-html, marp-bizcase-*, and marp-onepager-pdf.
• scripts/require-bizcase-slides.sh guard behavior for gitignored local business-case sources.
• .github/workflows/ci.yml and .pre-commit-config.yaml validation flow.

Key knowledge gaps addressed

• Removed stale 19-slide references after the deck grew to 26 slides.
• Clarified that CI runs full-history gitleaks plus pre-commit gitleaks/shellcheck/markdownlint.
• Documented how private demo-slides/claude-enterprise-* sources and exports should remain local and gitignored.

Validation

• pre-commit run --all-files