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/opencode/opencode_test.go

@@ -129,15 +129,15 @@ func TestOpencodeRenderProviderHasNpm(t *testing.T) {
 	// provider map has npm field.
 	provider, ok := parsed["provider"].(map[string]any)
 	require.True(t, ok)
-	gw, ok := provider["harness-sync-gateway"].(map[string]any)
+	gw, ok := provider["hs-gw"].(map[string]any)
 	require.True(t, ok)
 	assert.Equal(t, "@ai-sdk/openai-compatible", gw["npm"])
 	opts, ok := gw["options"].(map[string]any)
 	require.True(t, ok)
 	assert.Equal(t, "https://gw", opts["baseURL"])
 
-	// model is "harness-sync-gateway/modelID".
-	assert.Equal(t, "harness-sync-gateway/claude-sonnet-4-6", parsed["model"])
+	// model is "hs-gw/modelID".
+	assert.Equal(t, "hs-gw/claude-sonnet-4-6", parsed["model"])
 }
 
 func TestOpencodeImportStripsComments(t *testing.T) {