Add canonical technical skill pages

[yawboakye]

Summary

• add canonical /technical-skills/:slug pages to the developer directory
• curate function-first descriptions, official links, source links, why-it-matters copy and company references for all 60 skills currently exposed by the public directory
• keep uncurated skills renderable with category-specific fallback copy while marking them noindex and excluding them from the sitemap until curated
• link profile and project skill badges to the canonical skill pages

Data and SEO

• seed stable categories and curated metadata idempotently
• preserve distinct slugs for names such as C, C# and C++
• add canonical titles, descriptions and URLs for skill-filtered directory pages
• include only curated skill pages in sitemap.xml

Acceptance criteria

Implemented in 8acc036:

• A skill-filtered directory visibly identifies the selected skill without relying on the form control. (skill heading and introduction)
• The page includes a useful canonical description covering what the skill is, its common uses, and why it matters. (curated React content, canonical content accessors)
• Skill content comes from a single maintainable source of truth. (centralised YAML content loaded idempotently into skills)
• Empty results still show the skill introduction. (request coverage for a skill page with no matches)
• Page title and metadata identify the selected skill. (title, description and canonical URL)
• Unknown skills have an intentional error or fallback state. (strict skill lookup, 404 coverage for canonical and query-parameter URLs)
• The existing skill filter continues to work. (filter implementation, integration coverage)
• Request or integration coverage verifies the skill heading, description, metadata, empty state, and invalid-skill behavior. (skill-page and invalid-skill integration tests)

Testing

• focused skill/public-site/sitemap suite: 21 tests, 1,133 assertions, 0 failures
• StandardRB
• Zeitwerk check
• whitespace check
• full suite: 243 tests, 0 failures and 4 errors because native Vips::Image is unavailable for the existing OG-card tests in this local environment

Screenshots

Hotwire

PostgreSQL

Closes #452