fix(opencode-go): keep users on opencode-go instead of hijacking to native providers (#20802)

OpenCode Go and OpenCode Zen are flat-namespace model resellers — their
/v1/models returns bare IDs (deepseek-v4-flash, minimax-m2.7), and the
inference API rejects vendor-prefixed names with HTTP 401 'Model not
supported'. Two bugs fixed:

1. `switch_model` in hermes_cli/model_switch.py was silently switching the
   user off opencode-go to native deepseek when they typed
   `/model deepseek-v4-flash`. Step d found the model in opencode-go's live
   catalog, but step e (detect_provider_for_model) still ran and matched
   the bare name against deepseek's static catalog. Fix: track whether
   the live catalog resolved it; skip step e when it did.

2. `normalize_model_for_provider` in hermes_cli/model_normalize.py only
   stripped the exact `opencode-zen/` prefix, leaving arbitrary vendor
   prefixes like `minimax/minimax-m2.7` (commonly copied from aggregator
   slugs into fallback_model configs) intact — causing HTTP 401s when
   the fallback chain activated. Fix: opencode-go/opencode-zen strip ANY
   leading vendor prefix because their APIs are flat-namespace.

Tests: 11 new cases in tests/hermes_cli/test_opencode_go_flat_namespace.py
covering both normalization (prefix stripping, regression guards for
opencode-zen Claude hyphenation and openrouter vendor-prepending) and
switch_model (bare-name resolution on opencode-go's live catalog must
not trigger cross-provider hijack).

Reported by @Ufonik via Discord; Kimi K2.6 always worked because moonshotai
has no overlapping entry in a native provider's static catalog. Deepseek
and minimax failed because their v4/v2.7 names existed in the native
deepseek/minimax catalogs.

hermes_cli/model_switch.py

@@ -799,6 +799,12 @@ def switch_model(
                         )
 
         # --- Step d: Aggregator catalog search ---
+        # Track whether the live catalog of the CURRENT provider resolved the
+        # model — if so, step e must not second-guess and switch providers.
+        # Critical for flat-namespace resellers like opencode-go / opencode-zen
+        # whose live /v1/models returns bare IDs (e.g. "deepseek-v4-flash") that
+        # coincidentally match entries in native providers' static catalogs.
+        resolved_in_current_catalog = False
         if is_aggregator(target_provider) and not resolved_alias:
             catalog = list_provider_models(target_provider)
             if catalog:
@@ -806,6 +812,7 @@ def switch_model(
                 for mid in catalog:
                     if mid.lower() == new_model_lower:
                         new_model = mid
+                        resolved_in_current_catalog = True
                         break
                 else:
                     for mid in catalog:
@@ -813,6 +820,7 @@ def switch_model(
                             _, bare = mid.split("/", 1)
                             if bare.lower() == new_model_lower:
                                 new_model = mid
+                                resolved_in_current_catalog = True
                                 break
 
         # --- Step e: detect_provider_for_model() as last resort ---
@@ -825,6 +833,7 @@ def switch_model(
             target_provider == current_provider
             and not is_custom
             and not resolved_alias
+            and not resolved_in_current_catalog
         ):
             detected = detect_provider_for_model(new_model, current_provider)
             if detected: