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, 11 insertions, 5 deletions

diff --git a/src/tui_selectors.zig b/src/tui_selectors.zig
index a37acb8..590d321 100644
--- a/src/tui_selectors.zig
+++ b/src/tui_selectors.zig
@@ -48,7 +48,10 @@ pub const ReasoningOption = struct {
     detail: []const u8,
     value: Value,
 
-    pub const Value = union(APIStyle) {
+    /// Two payload families, decoupled from `APIStyle`: `openai_chat` carries
+    /// a `reasoning` effort (shared by the `openai_chat` and `openai_responses`
+    /// styles), `anthropic_messages` carries a thinking selection.
+    pub const Value = union(enum) {
         /// OpenAI `reasoning` enum value.
         openai_chat: ReasoningEffort,
         /// Anthropic thinking selection: either disabled, or adaptive at an
@@ -85,7 +88,8 @@ pub const ReasoningOption = struct {
     /// The reasoning options offered for a provider config's API style.
     pub fn forStyle(style: APIStyle) []const ReasoningOption {
         return switch (style) {
-            .openai_chat => &openai,
+            // The Responses style takes the same reasoning-effort options.
+            .openai_chat, .openai_responses => &openai,
             .anthropic_messages => &anthropic,
         };
     }
@@ -95,6 +99,7 @@ pub const ReasoningOption = struct {
         switch (self.value) {
             .openai_chat => |r| switch (cfg.*) {
                 .openai_chat => |*c| c.reasoning = r,
+                .openai_responses => |*c| c.reasoning = r,
                 .anthropic_messages => {},
             },
             .anthropic_messages => |sel| switch (cfg.*) {
@@ -105,7 +110,7 @@ pub const ReasoningOption = struct {
                         c.effort = e;
                     },
                 },
-                .openai_chat => {},
+                .openai_chat, .openai_responses => {},
             },
         }
     }
@@ -114,7 +119,8 @@ pub const ReasoningOption = struct {
     /// preselect the picker on the active value).
     pub fn matches(self: ReasoningOption, cfg: ProviderConfig) bool {
         switch (self.value) {
-            .openai_chat => |r| return cfg == .openai_chat and cfg.openai_chat.reasoning == r,
+            .openai_chat => |r| return (cfg == .openai_chat and cfg.openai_chat.reasoning == r) or
+                (cfg == .openai_responses and cfg.openai_responses.reasoning == r),
             .anthropic_messages => |sel| {
                 if (cfg != .anthropic_messages) return false;
                 const c = cfg.anthropic_messages;
@@ -167,7 +173,7 @@ pub fn adaptiveFallback(cfg: *ProviderConfig) bool {
             c.thinking_budget_tokens = budget;
             return true;
         },
-        .openai_chat => return false,
+        .openai_chat, .openai_responses => return false,
     }
 }