chore: migrate repository URLs from AnsoCode/Sencho to studio-saelix/sencho

Updates all hardcoded GitHub repository references across 21 files:
- package.json: repository URL, bugs URL, homepage, description, author
- CONTRIBUTING.md: bug report template URL
- SECURITY.md: advisory URL, cosign cert-identity regexp
- .github/CODEOWNERS: @AnsoCode -> @studio-saelix/maintainers
- .github/workflows/ci.yml: repositories scope (Sencho -> sencho), docs-sync git URL
- .github/workflows/cla.yml: path-to-document URL
- .github/workflows/docker-publish.yml: cosign verify comment
- frontend/**/*.tsx: issues and changelog links (3 components)
- frontend/public/.well-known/security.txt: Contact and Policy URLs
- security/vex/sencho.openvex.json: @id field
- docs/openapi.yaml: license URL
- docs/docs.json: navbar and footer GitHub links (5 instances)
- docs/security.mdx: advisory and SECURITY.md links
- docs/reference/verifying-images.mdx: repo link + cosign regexp + legacy identity note
- docs/reference/contact.mdx: issues, LICENSE, advisory, policy, CoC links
- docs/reference/security-advisories.mdx: releases link
- docs/operations/verifying-images.mdx: cosign regexps and VEX download URL (6 instances)
- docs/operations/upgrade.mdx: releases links (2 instances)
- backend/src/utils/version-check.ts: GitHub Releases API endpoint

CHANGELOG.md intentionally excluded (release-please managed).
Legacy cosign identity note added for pre-migration image verification.

.github/workflows/cla.yml

@@ -24,6 +24,6 @@ jobs:
           # PERSONAL_ACCESS_TOKEN: ${{ secrets.PERSONAL_ACCESS_TOKEN }}
         with:
           path-to-signatures: 'signatures/version1/cla.json'
-          path-to-document: 'https://github.com/AnsoCode/Sencho/blob/main/CLA.md'
+          path-to-document: 'https://github.com/studio-saelix/sencho/blob/main/CLA.md'
           branch: 'main'
           allowlist: 'dependabot[bot],greenkeeper[bot],github-actions[bot],release-please[bot],sencho-quartermaster[bot]'