auth: openai_responses provider for Codex subscription (Tier 2)

Add the OpenAI Responses API wire dialect (openai_responses APIStyle +
OpenAIResponsesConfig), a request serializer (instructions/input items,
flat function tools, reasoning, store:false, include encrypted reasoning),
and a streaming state machine over the typed response.* SSE events mapping
to the shared block model. Wire dispatch, buildProviderConfig, the C-ABI
free switch, and the reasoning selectors. Ship a commented Codex example in
the default config.

Implemented from the Responses API docs + the open-source Codex client and
covered by fixture tests; live verification against a ChatGPT plan is still
required (documented).

1 files changed, 5 insertions, 0 deletions

diff --git a/libpanto/src/public.zig b/libpanto/src/public.zig
index ec8534a..339c6f1 100644
--- a/libpanto/src/public.zig
+++ b/libpanto/src/public.zig
@@ -67,6 +67,7 @@ pub const Config = config_mod.Config;
 pub const ProviderConfig = config_mod.ProviderConfig;
 pub const OpenAIChatConfig = config_mod.OpenAIChatConfig;
 pub const AnthropicMessagesConfig = config_mod.AnthropicMessagesConfig;
+pub const OpenAIResponsesConfig = config_mod.OpenAIResponsesConfig;
 pub const APIStyle = config_mod.APIStyle;
 pub const ReasoningEffort = config_mod.ReasoningEffort;
 pub const Thinking = config_mod.Thinking;
@@ -236,6 +237,8 @@ const openai_chat_json = @import("openai_chat_json.zig");
 const provider_openai_chat = @import("provider_openai_chat.zig");
 const anthropic_messages_json = @import("anthropic_messages_json.zig");
 const provider_anthropic_messages = @import("provider_anthropic_messages.zig");
+const openai_responses_json = @import("openai_responses_json.zig");
+const provider_openai_responses = @import("provider_openai_responses.zig");
 const compaction_mod = @import("compaction.zig");
 const sse_mod = @import("sse.zig");
 const tool_registry_mod = @import("tool_registry.zig");
@@ -274,4 +277,6 @@ test {
     std.testing.refAllDecls(provider_openai_chat);
     std.testing.refAllDecls(anthropic_messages_json);
     std.testing.refAllDecls(provider_anthropic_messages);
+    std.testing.refAllDecls(openai_responses_json);
+    std.testing.refAllDecls(provider_openai_responses);
 }