Add clickable tag badges to plan cards and header

[HamptonMakes]

Summary

Tags are now visible and interactive in the UI. Clicking a tag filters the plan index to show only plans with that tag.

Changes

• Plan cards — Tag badges rendered below the header row, linking to ?tag=name
• Plan header — Tag badges shown after status/type badges
• Tag filtering — GET /plans?tag=infrastructure filters plans via Plan.with_tag scope
• Active filter bar — When filtering by tag, shows the active tag with a ✕ Clear link
• Filter preservation — Tag filter is preserved when switching scope, status, or plan type
• Seeds — Added 2 demo plans with tags (api, infrastructure, security, design, etc.) and tagged existing Q3 roadmap
• Specs — 4 new request specs covering tag filtering, badge rendering, and filter bar

To test locally

bundle exec rails db:seed
bundle exec rails s

Visit http://localhost:3000 — you should see tag badges on plan cards. Click one to filter.