Extends hermes_cli.xai_retirement with apply_migration(config_path,
issues, backup=True), used by the upcoming `hermes migrate xai`
sub-command.
Uses ruamel.yaml round-trip mode so that comments, key order,
indentation, quoting style, and scalar types are preserved on
rewrite — config.yaml is treated as a user-edited file, not a
data dump.
Behavior:
- Each issue rewrites parent[leaf] to issue.replacement
- When issue.reasoning_effort is set (non-reasoning variants
that map to grok-4.3), a sibling reasoning_effort key is
added/updated alongside the model
- Empty issues list or missing slots are no-ops (no backup,
no rewrite)
- When changes occur, a timestamped backup
(.bak-pre-migrate-xai-YYYYMMDD-HHMMSS) is written first
unless backup=False
17 unit tests cover dry-run/no-op, surgical replacement (each
slot), comment + key-order preservation, backup creation, and
idempotence (apply twice → no-op the second time).
