feat(cli): add 'hermes fallback' command to manage fallback providers (#16052)

Manage the fallback_providers chain from the CLI instead of hand-editing
config.yaml. The picker reuses select_provider_and_model() from 'hermes
model' — same provider list, same credential prompts, same model picker.

  hermes fallback [list]   Show the current chain (primary + fallbacks)
  hermes fallback add      Run the model picker, append selection to chain
  hermes fallback remove   Pick an entry to delete (arrow-key menu)
  hermes fallback clear    Remove all entries (with confirmation)

'add' snapshots config['model'] before calling the picker, extracts the
user's selection from the post-picker state, then restores the primary
and appends {provider, model, base_url?, api_mode?} to fallback_providers.
Auth store's active_provider is snapshot/restored too so OAuth-provider
fallbacks don't silently deactivate the user's primary. Duplicates and
self-as-fallback are rejected. Legacy single-dict 'fallback_model' entries
are auto-migrated to the list format on first write.

hermes_cli/main.py

@@ -7223,6 +7223,9 @@ Examples:
     hermes auth remove <p> <t>    Remove pooled credential by index, id, or label
     hermes auth reset <provider>  Clear exhaustion status for a provider
     hermes model                  Select default model
+    hermes fallback [list]        Show fallback provider chain
+    hermes fallback add           Add a fallback provider (same picker as `hermes model`)
+    hermes fallback remove        Remove a fallback provider from the chain
     hermes config                 View configuration
     hermes config edit            Edit config in $EDITOR
     hermes config set model gpt-4 Set a config value
@@ -7564,6 +7567,42 @@ For more help on a command:
     )
     model_parser.set_defaults(func=cmd_model)
 
+    # =========================================================================
+    # fallback command — manage the fallback provider chain
+    # =========================================================================
+    from hermes_cli.fallback_cmd import cmd_fallback
+
+    fallback_parser = subparsers.add_parser(
+        "fallback",
+        help="Manage fallback providers (tried when the primary model fails)",
+        description=(
+            "Manage the fallback provider chain.  Fallback providers are tried "
+            "in order when the primary model fails with rate-limit, overload, or "
+            "connection errors.  See: "
+            "https://hermes-agent.nousresearch.com/docs/user-guide/features/fallback-providers"
+        ),
+    )
+    fallback_subparsers = fallback_parser.add_subparsers(dest="fallback_command")
+    fallback_subparsers.add_parser(
+        "list",
+        aliases=["ls"],
+        help="Show the current fallback chain (default when no subcommand)",
+    )
+    fallback_subparsers.add_parser(
+        "add",
+        help="Pick a provider + model (same picker as `hermes model`) and append to the chain",
+    )
+    fallback_subparsers.add_parser(
+        "remove",
+        aliases=["rm"],
+        help="Pick an entry to delete from the chain",
+    )
+    fallback_subparsers.add_parser(
+        "clear",
+        help="Remove all fallback entries",
+    )
+    fallback_parser.set_defaults(func=cmd_fallback)
+
     # =========================================================================
     # gateway command
     # =========================================================================