Polish GitHub Pages for public launch

[ldraney]

Summary

Prepares the GitHub Pages site for public launch with deployment automation, updated content, and consistent navigation.

• Add deploy-pages.yml workflow for automatic deployment on push to main
• Update architecture.html component status to reflect shipped state
• Add consistent navigation (back link + footer) to all docs/ pages
• Add favicon to all HTML pages
• Create 404.html error page
• Add .nojekyll to ensure JS works correctly
• Add docs link to main page footer

Changes

File	Change
.github/workflows/deploy-pages.yml	New workflow for Pages deployment
.nojekyll	Prevent Jekyll processing
404.html	Custom error page
index.html	Add favicon, add docs link to footer
security.html	Add favicon
docs/architecture.html	Update status table, add nav/footer, add favicon
docs/roadmap.html	Add nav/footer, add favicon
docs/user-story.html	Add nav/footer, add favicon

Post-merge action required

After merging, enable GitHub Pages in repo settings:
Settings → Pages → Source: GitHub Actions

Test plan

• Verify workflow runs on merge
• Check site loads at https://ldraney.github.io/obsidian-mcp/
• Verify navigation works between all pages
• Confirm favicon appears in browser tabs
• Test 404 page by visiting invalid URL

Closes #70

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