Config path values are not tilde- or environment-expanded #80

Surfaced during the 2026-07-27 documentation audit; verified empirically against a built binary.

directories.package, directories.target, symlinks.backup_dir, and logging.file are stored literally and resolved against the working directory. A config containing target: /tildetest plans 'Create directory: //tildetest/p1', and target: $HOME plans '/$HOME/p1'; both create a literal directory named '' or '$HOME' under the current directory. Only pkg/dot/adopt_service.go:94 expands '', and only for adopt arguments. 'target: ' alone appears to work only because bare '' is YAML null, so the default applies.

Consequence: a repository config that pins directories with ~ or $HOME silently mislocates every operation, and the same config cannot be portable across machines without absolute paths.

Suggested fix: expand a leading ~ and environment variables when loading path-typed config values (directories.*, symlinks.backup_dir, logging.file). Docs were updated to use absolute paths and state the current rule.