Add PostGIS skill with upstream sync checking

[cevian]

Summary

• Add PostGIS skill imported from gitea.osgeo.org/postgis/postgis, trimmed to PostGIS-specific content
• Add upstream-skills.json to track upstream sources with author attribution
• Add check script (npm run check:upstream-skills) to detect upstream changes
• Add GitHub Actions workflow to run weekly checks and create issues when upstream changes

Attribution

Thanks to Darafei Praliaskouski (@komzpa) for creating and maintaining the original PostGIS SKILL.md.

Test plan

• [x] npm run check:upstream-skills runs successfully
• [x] npm run lint passes
• [x] npm run prettier:check passes

🤖 Generated with Claude Code