Add a new section in run_doctor that lists retired xAI model
references found in the active config and points the user at the
official xAI migration guide.

Each retired reference shows its config path (principal.model,
auxiliary.<slot>.model, delegation.model, tts.xai.model, or
plugins.image_gen.xai.model), the recommended replacement, and
whether reasoning_effort needs to be set (for non-reasoning variants
that map to grok-4.3 + reasoning_effort=none).

Findings are appended to manual_issues so the final doctor summary
reminds the user to update their config.yaml manually (no automatic
YAML rewriting in this PR — preserves comments, key order, types).

Wrapped in try/except so doctor still completes if load_config or
the retirement module raise unexpectedly.

Print a non-blocking stderr warning at the top of cmd_chat when the
active config still references xAI models scheduled for retirement
on May 15, 2026. Each line includes the config path, the recommended
replacement, and the reasoning_effort to set for non-reasoning
variants. Points to hermes doctor for full diagnostic.

Wrapped in try/except — never blocks startup. After May 15 the
upstream xAI API will return a clear error anyway; this is purely a
heads-up to give users time to migrate before that happens.

…round-trip

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).

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.