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.

package.json

@@ -1,7 +1,7 @@
 {
   "name": "sencho",
   "version": "0.65.0",
-  "description": "",
+  "description": "Self-hosted Docker control plane for Compose with atomic deployments, self-healing, and fleet-wide automation.",
   "main": "index.js",
   "scripts": {
     "test": "cd backend && npm test",
@@ -10,18 +10,18 @@
   },
   "repository": {
     "type": "git",
-    "url": "git+https://github.com/AnsoCode/Sencho.git"
+    "url": "git+https://github.com/studio-saelix/sencho.git"
   },
   "keywords": [],
-  "author": "",
+  "author": "Studio Saelix",
   "license": "SEE LICENSE IN LICENSE",
   "type": "commonjs",
   "bugs": {
-    "url": "https://github.com/AnsoCode/Sencho/issues"
+    "url": "https://github.com/studio-saelix/sencho/issues"
   },
-  "homepage": "https://github.com/AnsoCode/Sencho#readme",
+  "homepage": "https://github.com/studio-saelix/sencho#readme",
   "devDependencies": {
     "@playwright/test": "^1.59.1",
     "otplib": "^13.4.0"
   }
-}
+}