feat(cli): hermes migrate xai [--apply] [--no-backup]

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.

hermes_cli/migrate.py

@@ -0,0 +1,115 @@
+"""CLI handlers for ``hermes migrate ...``.
+
+Currently exposes only ``hermes migrate xai`` — diagnoses and (with --apply)
+rewrites references to xAI models retired on May 15, 2026.
+"""
+from __future__ import annotations
+
+import sys
+from pathlib import Path
+from typing import Any
+
+from hermes_cli.colors import Colors, color
+from hermes_cli.config import load_config
+
+
+def cmd_migrate(args: Any) -> int:
+    """Dispatcher for ``hermes migrate <subtype>``."""
+    sub = getattr(args, "migrate_type", None)
+    if sub == "xai":
+        return cmd_migrate_xai(args)
+
+    print("usage: hermes migrate xai [--apply] [--no-backup]", file=sys.stderr)
+    return 2
+
+
+def cmd_migrate_xai(args: Any) -> int:
+    """Run xAI May-15 model migration in dry-run or apply mode."""
+    from hermes_cli.xai_retirement import (
+        MIGRATION_GUIDE_URL,
+        RETIREMENT_DATE,
+        apply_migration,
+        find_retired_xai_refs,
+        format_issue,
+    )
+
+    apply = bool(getattr(args, "apply", False))
+    no_backup = bool(getattr(args, "no_backup", False))
+
+    config = load_config()
+    issues = find_retired_xai_refs(config)
+
+    print()
+    print(color(
+        f"◆ xAI Model Retirement Migration ({RETIREMENT_DATE})",
+        Colors.CYAN, Colors.BOLD,
+    ))
+    print()
+
+    if not issues:
+        print(f"  {color('✓', Colors.GREEN)} No retired xAI models in config — nothing to migrate.")
+        return 0
+
+    print(f"  Found {len(issues)} retired xAI model reference(s):")
+    print()
+    for issue in issues:
+        print(f"    {color('⚠', Colors.YELLOW)} {format_issue(issue)}")
+    print()
+    print(f"    {color('→', Colors.CYAN)} Migration guide: {MIGRATION_GUIDE_URL}")
+    print()
+
+    config_path = _resolve_config_path()
+
+    if not apply:
+        print(color("Dry-run mode — no changes written.", Colors.DIM))
+        print(color(
+            "Re-run with `hermes migrate xai --apply` to rewrite "
+            f"{config_path} in-place (backup created automatically).",
+            Colors.DIM,
+        ))
+        return 0
+
+    if not config_path or not config_path.exists():
+        print(
+            f"  {color('✗', Colors.RED)} Could not locate config.yaml "
+            f"(looked at: {config_path})",
+            file=sys.stderr,
+        )
+        return 1
+
+    try:
+        result = apply_migration(
+            config_path=config_path,
+            issues=issues,
+            backup=not no_backup,
+        )
+    except Exception as exc:
+        print(
+            f"  {color('✗', Colors.RED)} Migration failed: {exc}",
+            file=sys.stderr,
+        )
+        return 1
+
+    if not result.config_changed:
+        print(f"  {color('⚠', Colors.YELLOW)} No changes written.")
+        return 0
+
+    if result.backup_path is not None:
+        print(f"  {color('✓', Colors.GREEN)} Backup: {result.backup_path}")
+    print(
+        f"  {color('✓', Colors.GREEN)} Updated {len(result.issues_resolved)} "
+        f"slot(s) in {result.file_path}"
+    )
+    print()
+    print(color(
+        "Run `hermes doctor` to confirm no retired xAI models remain.",
+        Colors.DIM,
+    ))
+    return 0
+
+
+def _resolve_config_path() -> Path:
+    """Best-effort: locate the active config.yaml on disk."""
+    from hermes_cli.config import get_hermes_home
+
+    return get_hermes_home() / "config.yaml"