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/src/main.zig b/src/main.zig
index 61a88bd..2887730 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -358,7 +358,7 @@ pub fn main(init: std.process.Init) !void {
     // On resume, reconstruct the conversation from the store. (The
     // dangling-prompt recovery feature was dropped in R2.) On a fresh
     // session, the agent starts with an empty conversation.
-    var adopted_conversation: ?panto.ConversationData = null;
+    var adopted_conversation: ?panto.Conversation = null;
     if (is_resume) {
         adopted_conversation = try session.load();
         const sid = session.info.id;