feat(sync): persist ldcli installation identity

[ctawiah]

Summary

Persists a stable installation ID in the existing ldcli user configuration for non-Git source identity. This is a focused configuration layer above Git source detection.

• Creates a random installation ID only when one does not already exist.
• Writes the config atomically with restrictive permissions and preserves existing and unknown settings.
• Keeps the internal ID out of ldcli config --list output.
• Adds no metadata file to the customer workspace.

Verification

• go test ./internal/config -count=1
• Focused lint for ./internal/config

[ctawiah]

Consolidated with the other automatic source identity work in #789.