Update uGig social adapter to posts API

[Photon101]

Summary

• replace the stale uGig prompts-marketplace integration with the current documented /api/posts community feed API
• support UGIG_API_KEY via X-API-Key, while preserving legacy UGIG_TOKEN bearer auth fallback
• map sh1pt social posts to uGig content, url, post_type, and tags, returning the public /post/:id URL
• expand tests from a smoke check to contract, auth, request-shape, fallback-token, and API-error coverage

Validation

• corepack pnpm exec vitest run packages/social/ugig/src/index.test.ts
• corepack pnpm exec tsc -p packages/social/ugig/tsconfig.json --noEmit
• git diff --check