Move registry cache to XDG cache directory

[fuzzyporpoise]

Summary

Adds a machine-local project registry at $XDG_CACHE_HOME/lnk/registry.json, moving it out of the lnk repo so relocating or renaming the repo no longer orphans the mapping. Complements the existing project-scope work.

What's new

• XDG registry - the project cache moves from <repo>/.lnkprojectcache to $XDG_CACHE_HOME/lnk/registry.json, falling back to ~/.cache/lnk/registry.json and then the OS temp directory.
• LNK_REGISTRY override - redirect the registry file anywhere, matching the TODO_REGISTRY escape hatch in the todo CLI.
• Atomic saves - RegistryPath() resolves the location, the cache directory is created on demand, and writes go through a temp file plus rename.
• Docs and help text - project cache, project sync --all, and the doctor messages now describe the registry; README and AGENTS.md are updated.

Behavior changes / guardrails

• No automatic migration: an existing <repo>/.lnkprojectcache is ignored; run lnk project cache --scan <dir> to rebuild the registry.
• The registry is never written into the lnk repo, so it cannot be committed or synced across machines.
• The on-disk schema is unchanged ({"projects":[...]}), so consumers only need the new path.

Testing

• Unit tests cover RegistryPath() (override, XDG, home fallback) and that saves create the registry directory and leave no temp file behind.
• Project cache, sync --all, and doctor tests resolve the registry through service.RegistryPath(), and TestHome pins XDG_CACHE_HOME for hermetic runs.
• make check runs fmt, vet, lint, unit, and integration suites; all green.