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, 1 insertions, 1 deletions

diff --git a/libpanto/src/file_system_jsonl_store.zig b/libpanto/src/file_system_jsonl_store.zig
index 10b65b3..bbf8ea5 100644
--- a/libpanto/src/file_system_jsonl_store.zig
+++ b/libpanto/src/file_system_jsonl_store.zig
@@ -2055,7 +2055,7 @@ test "FileSystemJSONLStore catalog: create → append → load round-trips" {
     var conv = conversation_mod.Conversation.init(testing.allocator);
     defer conv.deinit();
     try conv.addUserMessage("ping");
-    try conv.addAssistantMessage(&.{});
+    try conv.addAssistantMessage(&.{}, null);
 
     const id: session_store_mod.WireIdentity = .{ .api_style = .openai_chat, .base_url = "u", .model = "m" };
     var batch = [_]session_store_mod.PersistentMessage{