v0.6.0

feat: add restic support, rename to backup-exporter

Adds a second collector for restic-backed repositories alongside borg,
with full parity including per-snapshot history metrics (restic_*).
Restructures the exporter around a tool-agnostic Collector interface,
two independent collection schedulers (borg/restic), and a shared
exporter_* metric family for collection-health/system-info that
replaces the old borg_last_collect_*/borg_system_info metrics.

Config moves from flags/BORG_REPOSITORIES to a YAML file listing
repositories with a type discriminant, supporting restic's
password/password_file/password_command auth conventions. This is a
breaking change with no compat shim.

Repo, Go module and binary renamed borg-exporter -> backup-exporter to
reflect the multi-tool scope.

Fixes a pre-existing bug surfaced while restructuring: repo.Opts was
passed to exec.Command as a single argument instead of being split,
so multi-flag borg opts (e.g. "--lock-wait 5") would have been sent
to borg malformed.

Closes #8