Define typed symlink creation in a future plan schema version #5

Version 1 now intentionally supports only pre-existing symlinks identified by base Git tree mode and tracked through renames. Design a future schema version with typed declarations and target constraints before allowing new symlinks or regular-file-to-link conversions. Retain immutable v1 snapshots, add explicit migrations, and cover import plus post-run enforcement. See PR #1 review discussion 4078629685. This is outside the current v1 implementation scope.