feat(sync): add local catalog fallback

[AkashKumar7902]

Summary

• add localCatalogFallback for sync registries when upstream catalog listing fails
• enumerate locally cached repositories and map destination/stripPrefix repos back to upstream names
• document the new sync option in the examples

Tests

• GOEXPERIMENT=jsonv2 go test -tags "sync scrub metrics search lint" ./pkg/extensions/sync -run TestGetNextRepoLocalCatalogFallback -count=1
• GOEXPERIMENT=jsonv2 go test -race -tags "sync scrub metrics search lint" ./pkg/extensions/sync -run TestGetNextRepoLocalCatalogFallback -count=1
• GOEXPERIMENT=jsonv2 go test -tags "sync scrub metrics search lint" ./pkg/extensions/sync -run '^$' -count=1
• GOEXPERIMENT=jsonv2 go test ./pkg/cli/server -run '^$' -count=1
• GOEXPERIMENT=jsonv2 go test -tags "sync scrub metrics search lint" ./pkg/extensions/config/sync -count=1
• jq empty examples/config-sync.json
• git diff --check

Fixes #3843