Fix data-loss bugs and make adopt write to the config

[youhide]

Four defects that either destroyed files or failed silently:

• defaults.link without a backup: key disabled backups entirely (Go zero value), so force: true overwrote real files with no copy. The LinkDefaults fields are now *bool, absent means backup enabled, and a failed backup aborts the overwrite instead of proceeding.
• Duplicate detection deleted any symlink in the target's directory pointing at the same source, so two config entries sharing a source destroyed each other on every run. It is now opt-in via remove_duplicates and never touches a declared target.
• Runs exited 0 even when every link failed. Commands now end with failureError(), and SilenceUsage keeps runtime errors readable.
• Template execution errors were swallowed, creating links with {{ .Hostnam }} in their names. Exec errors are now fatal; parse errors stay tolerated for configs holding a literal {{.

adopt now finishes the job: it takes several paths, mirrors each one's location inside the repo minus leading dots (~/.config/nvim -> config/nvim), and inserts the link entry into the config through a yaml.Node round-trip that preserves comments. It edits the raw file rather than the template-expanded copy, so {{ }} variables survive, and falls back to printing the entry when the config cannot be parsed or no profile section matches. Adds --to and --no-config.

Also: deterministic processing order (maps were iterated at random), failing pre_* hooks abort their section, a backup manifest so backup list shows real paths and dates, stale directory backups are cleared before recopying, a warning when there is no backup to restore, and --quiet no longer prints the summary.

Tests cover the destructive paths for the first time: createLink, duplicate handling, adopt, config insertion, unlink --restore and exit code propagation.