Show host profile install status in lnk list --all

[polymorcodeus]

What's new

• lnk list --all now tags each host scope with [active] or [not installed] by checking whether any managed symlink exists on the current machine. Common scope is always active.
• No configuration, marker files, or migration needed -- detection is entirely automatic.

Usage

$ lnk list --all
common:
  .bashrc
  .gitconfig

personal [active]:
  .ssh/config

work [not installed]:
  .gitconfig.work

Testing

• service/list_test.go -- 6 new tests covering active host, inactive host, common always active, host-scope flag, and empty host scope.
• Full make check (fmt, vet, lint, unit tests, integration tests) passes.

Closes #6.