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/provider_anthropic_messages.zig b/libpanto/src/provider_anthropic_messages.zig
index 4bd146a..d76793b 100644
--- a/libpanto/src/provider_anthropic_messages.zig
+++ b/libpanto/src/provider_anthropic_messages.zig
@@ -502,7 +502,7 @@ const StreamState = struct {
         defer self.allocator.free(moved_blocks);
 
         const usage: ?provider_mod.Usage = if (self.usage_seen) self.usage else null;
-        try conv.addAssistantMessageWithUsage(moved_blocks, usage);
+        try conv.addAssistantMessage(moved_blocks, usage);
 
         const msg = conv.messages.items[conv.messages.items.len - 1];
         try out.push(.{ .message_complete = .{ .message = msg, .usage = usage } });