refactor(importers): route home lookup through paths

[upsetbit]

Closes #80.

Summary

• add paths.UserHome() as the single wrapper around os.UserHomeDir
• route importer DefaultRoots() through paths.UserHome()
• route schedule home paths through paths.UserHome() and cover path fallback behavior

Local validation

• go test ./internal/paths ./internal/importers/... ./internal/cli/schedule
• devbox run -- just lint
• devbox run -- just test-race

---

^{Need help on this PR? Tag /codesmith with what you need. Autofix is disabled.}