Alias Conversation instead of facading it; merge addAssistantMessage

A facade that forwards every method 1:1 is worse than paring the real type
down to the intended surface and aliasing it. conversation.Conversation's
interface is already the public surface we want (init/deinit, the
add*/replace* builders, and messages/allocator as plain data fields), so
public.zig now aliases it straight through and Agent.conversation() returns
a borrowed *Conversation for in-place surgery. Drops the ConversationData
alias and the pointer-wrapper.

Merged addAssistantMessageWithUsage into addAssistantMessage(blocks, ?usage)
on the real type (separate method deleted), so the alias has the intended
one-method shape; all call sites updated.

Only Agent and Stream remain true facades -- they have genuinely-internal
fields plus API-shaping renames (Stream.phase->state, Phase->State) an alias
can't express.

1 files changed, 3 insertions, 3 deletions

diff --git a/libpanto/src/openai_chat_json.zig b/libpanto/src/openai_chat_json.zig
index 2307ce8..53d4b0c 100644
--- a/libpanto/src/openai_chat_json.zig
+++ b/libpanto/src/openai_chat_json.zig
@@ -640,7 +640,7 @@ test "serializeRequest - assistant Thinking blocks are stripped from outbound hi
     try conv.addAssistantMessage(&.{
         .{ .Thinking = .{ .text = try conversation.textualBlockFromSlice(allocator, "thinking step") } },
         .{ .Text = try conversation.textualBlockFromSlice(allocator, "answer here") },
-    });
+    }, null);
 
     const cfg = testConfig("gpt-4o");
     var tools = emptyTools();
@@ -865,7 +865,7 @@ test "serializeRequest - assistant ToolUse becomes tool_calls" {
     try conv.addAssistantMessage(&.{
         .{ .Text = try conversation.textualBlockFromSlice(allocator, "calling tool") },
         .{ .ToolUse = .{ .id = id, .name = name, .input = args } },
-    });
+    }, null);
 
     var tools = emptyTools();
     defer tools.deinit();
@@ -1053,7 +1053,7 @@ test "serializeRequest - compaction summary trims superseded prefix" {
     try conv.addUserMessage("ancient question");
     try conv.addAssistantMessage(&.{
         .{ .Text = try conversation.textualBlockFromSlice(allocator, "ancient answer") },
-    });
+    }, null);
     // Compaction summary resets conversation context.
     try conv.addCompactionSummary("summary of the ancient exchange");
     // Kept-verbatim suffix replayed after the summary.