feat: URL-derived provider keys (hs-<hostname>) + dedup providers sharing same gateway URL

- GatewayProviderKey() derives stable key from gateway URL hostname
  e.g. https://llmgw.h.raczylo.com/v1 → hs-llmgw-h-raczylo-com
- AbsorbDuplicateProviders() merges providers sharing the same baseURL
  into our canonical entry; user-crafted model metadata wins for overlaps
- kilo: renders to opencode.jsonc (primary config), deep-merges providers,
  deletes enabled_providers filter that was hiding our gateway
- opencode: absorbs duplicate providers from existing config
- All adapters use GatewayProviderKey() instead of hardcoded string
- Fix goose URL doubling (was /v1/v1/chat/completions → /chat/completions)
- Fix token propagation: api_key written directly (not env var ref)
- Zed: adds prompt_cache_key capability, deletes context_servers key
  to avoid ContextServerSettingsContent serde untagged-enum parse error

internal/adapters/zed/zed.go

@@ -136,7 +136,7 @@ func zedLanguageModels(p *canonical.Profile) map[string]any {
 	}
 	return map[string]any{
 		"openai_compatible": map[string]any{
-			common.GatewayProviderID: entry,
+			common.GatewayProviderKey(p.Gateway.URL): entry,
 		},
 	}
 }
@@ -156,7 +156,7 @@ func zedAgentBlock(existing []byte, p *canonical.Profile) map[string]any {
 	}
 	if p.Gateway.DefaultModel != "" {
 		base["default_model"] = map[string]any{
-			"provider": common.GatewayProviderID,
+			"provider": common.GatewayProviderKey(p.Gateway.URL),
 			"model":    p.Gateway.DefaultModel,
 		}
 	}