Add GitHub Actions docs workflow

[johnnygreco]

Summary

Adds a GitHub Actions workflow for the Zensical documentation site, removes the obsolete GitLab Pages CI file, and updates the README repository layout entry to point at the new GitHub workflow. The workflow validates documentation builds on every pull request and main push, and deploys site/ to GitHub Pages only from main or manual dispatch on main.

Closes #3

Validation

• test: python3 scripts/render-dev-notes.py -> passed
• lint: python3 -m py_compile scripts/render-dev-notes.py -> passed
• build: scripts/build-docs.sh -> passed; generated site/
• stale GitLab scan: rg -n "gitlab|GitLab|\.gitlab-ci|gitlab\.io" README.md docs zensical.toml scripts .github || true -> no matches
• local site smoke: python3 -m http.server 8000 --bind 127.0.0.1 --directory site plus curl -fsS checks -> HTTP 200 for /, /dev-notes/, /dev-notes/posts/2026-06-05-making-dev-notes-repeatable/, /research/runtime/, /assets/brand/favicon.svg, and /assets/brand/openshell-mark.svg
• generated content smoke: rg checks found Dev Notes cards, bylines, search index content, and brand asset references in generated output
• YAML syntax: Ruby YAML.load_file(".github/workflows/docs.yml") parsed the workflow and found pull_request, push, and workflow_dispatch events plus validate and deploy jobs
• GitHub Actions: PR head 14cf440f6274bd65f19024a9d084432cc7569e12 ran Docs; Validate documentation passed and Deploy documentation skipped for the pull request event as expected
• Actions lint: Docker image pull for rhysd/actionlint:latest failed due Docker Hub DNS/proxy resolution in the local environment, so the GitHub Actions run above is the authoritative workflow parser/execution check

Risks

GitHub Pages is not configured yet for this repo; tracker issue #5 covers enabling/configuring Pages and verifying the live deployed site after this workflow merges. The deploy job uses official GitHub-owned Pages actions, which are allowed by the repo's selected-actions policy.

Reviewer Notes

Reviewed through the goal-delivery independent panel process. Round 1 found one low-severity handoff issue: README.md still referenced the deleted .gitlab-ci.yml; this PR now fixes it. After the trigger simplification and README fix, Round 2 returned clean across robustness/correctness/security/operations, maintainability/supply-chain, and validation/adoption lenses.

Focus areas for any further review: workflow deploy gating and Pages permissions. Pull requests build but do not deploy; pushes or manual dispatches on main validate first, then build/package/deploy. Backward compatibility with GitLab CI was not required.