devlore must not know its callers: shed writ's accessors from cmd/internal/devlore #436

Epic: #430 · Feature: #405

The LAST item in windows-native-permissions.md -- last because
everything above it either protects a file or proves that protection, while this repairs the layering the
campaign disturbed, and it must not delay a security fix.

cmd/internal/devlore names the locations every tool shares, yet WritLayersDir() and WritReposDir()
are writ's alone -- so cmd/lore links a function describing writ's layer registry. A package every
command depends on for shared anchors must carry no per-command knowledge.

Exit: the writ-specific accessors live with writ; cmd/internal/devlore exposes only what more than one
command uses.