feat: add opt-in Sanity CMS integration with @sanity/client v7.x

[Copilot]

No CMS was wired up — all content lived as hardcoded TypeScript files in src/content/. This adds a fully opt-in Sanity integration: when SANITY_PROJECT_ID is set, content is fetched from Sanity at build time; otherwise the static files are used unchanged.

How it works

• scripts/fetch-content.ts — pre-build script that runs GROQ queries against Sanity and writes TypeScript files to src/content/_generated/, mirroring the exact export signatures of the static files
• vite.config.ts — adds a conditional alias: @/content/* → src/content/_generated/* when SANITY_PROJECT_ID is set; zero component code changes needed
• .github/workflows/deploy.yml — adds a conditional "Fetch content from Sanity" step (skipped when the secret is absent)
• sanity/schemas.ts — complete Sanity Studio schema definitions for all content types (siteSettings, pageContent, agendaTrack, resource, sponsorshipConfig, sponsor) ready to drop into a Studio project

Activation

Add three repository secrets and push:

Secret	Value
SANITY_PROJECT_ID	From sanity.io/manage
SANITY_DATASET	production
SANITY_TOKEN	Read-only API token

Content type mapping

Sanity document	Generates
siteSettings (singleton)	site.ts + agendaIntro field
pageContent (singleton)	sections.ts
agendaTrack (one per tab)	agenda.ts
resource (one per item)	resources.ts
sponsorshipConfig + sponsor	sponsors.ts