Adds a new `migrate` top-level sub-command that delegates to
`migrate xai` for now. xAI handler:
- Default: dry-run. Lists every retired xAI model reference
found in config.yaml, with the recommended replacement and
reasoning_effort hint, and points to the official xAI
migration guide.
- --apply: rewrites config.yaml in-place (via the ruamel
round-trip apply_migration helper from hermes_cli.xai_retirement).
A timestamped backup is created automatically.
- --no-backup: skips the backup when applying (opt-in only —
the safe default keeps a copy).
Together with the doctor + chat-startup warnings already in
this stack, this gives users three escalating signals before
the May 15, 2026 retirement date: green check / warning at
chat startup / actionable migration command.
