chore(docs): update GitHub Actions workflow for documentation deployment

[vchan-in]

Add GitHub Actions workflow to deploy Zensical docs to GitHub Pages automatically on push.

Changes

File	Change
.github/workflows/docs.yml	Added configure-pages@v5, updated checkout to @v5, upload-pages-artifact to @v4, added --clean flag to build

Testing

• npm run build passes
• npm test passes (all tests green)
• Manually tested in Extension Development Host

Workflow syntax validated. Triggers on pushes to main that touch docs/** or zensical.toml. Matches the official Zensical GitHub Pages deployment pattern from https://zensical.org/docs/publish-your-site/.

Documentation

• No docs change needed

Memory

• No new architectural decisions made

Checklist

• No new bare globalState string keys (used the constants file)
• No direct vscode.lm calls outside lm.ts
• markdownDescription used for any new settings (not description)
• Conventional commit message (docs: add GitHub Actions workflow for Zensical docs deployment)