Clean repository down to roadmap content and sync scripts

[arikchakma]

Summary

• Move src/data/roadmaps to a top level roadmaps/ directory, keeping only the per-roadmap content/ folders. The <slug>.json, <slug>.md, faqs.astro and migration-mapping.json files are dropped since the database is the source of truth for nodes, edges, seo, dimensions and faqs.
• Delete the Astro site, public/, packages/, tests/ and all app tooling. This repository is now only the roadmap content plus the three sync scripts the workflows drive.
• Add a top level lib/ with the helpers the scripts used to import from src/ (slugify, markdown to html, html to markdown, and the roadmap/topic types), so the scripts are self contained.
• cleanup-orphaned-content.ts no longer reads the deleted <slug>.json fallback or gates on <slug>.md frontmatter. It now keys off "has a content/ directory", which also fixes 17 roadmaps that were silently skipped because they never had a <slug>.md.
• Update the content path in sync-repo-to-database.yml, sync-repo-to-database.ts, the issue template config, contributing.md and readme.md. Remove deployment.yml and upgrade-dependencies.yml; the sync, cleanup, triage and ops workflows all stay.

Test plan

• pnpm install succeeds from a clean checkout
• npx tsc --noEmit passes
• npm run cleanup:orphaned-content -- --roadmap-slug=frontend runs against the live API and reports orphans without touching valid files
• npm run cleanup:orphaned-content -- --roadmap-slug=__all__ walks all 90 roadmaps, including ones with no <slug>.md
• Trigger "Sync to Repository" from the backoffice roadmap tree page and confirm the workflow dispatches and opens a PR writing to roadmaps/<slug>/content/
• Run the Sync Repo to Database workflow for one roadmap and confirm the topic content and resource links land unchanged in the database
• Confirm the contribution link on a roadmap topic resolves to a real file on GitHub (needs the paired web-roadmap change)