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, 9 insertions, 13 deletions

diff --git a/libpanto/src/conversation.zig b/libpanto/src/conversation.zig
index 1fc5016..a436378 100644
--- a/libpanto/src/conversation.zig
+++ b/libpanto/src/conversation.zig
@@ -292,15 +292,11 @@ pub const Conversation = struct {
         });
     }
 
-    /// Append an assistant message. Ownership of the blocks is transferred
-    /// to the conversation; the caller must not deinit them after this call.
-    pub fn addAssistantMessage(self: *Conversation, blocks: []const ContentBlock) !void {
-        return self.addAssistantMessageWithUsage(blocks, null);
-    }
-
-    /// Append an assistant message tagged with its provider-reported
-    /// `usage`. Ownership of the blocks is transferred to the conversation.
-    pub fn addAssistantMessageWithUsage(
+    /// Append an assistant message, optionally tagged with its
+    /// provider-reported `usage` (pass `null` for none). Ownership of the
+    /// blocks is transferred to the conversation; the caller must not
+    /// deinit them after this call.
+    pub fn addAssistantMessage(
         self: *Conversation,
         blocks: []const ContentBlock,
         usage: ?Usage,
@@ -415,7 +411,7 @@ test "Conversation - add messages and verify content" {
     try conv.addUserMessage("Hello!");
     try conv.addAssistantMessage(&.{
         .{ .Text = try textualBlockFromSlice(allocator, "Hi there!") },
-    });
+    }, null);
 
     try std.testing.expectEqual(@as(usize, 3), conv.messages.items.len);
 
@@ -455,7 +451,7 @@ test "Conversation - deinit frees without leaks" {
     try conv.addUserMessage("user message");
     try conv.addAssistantMessage(&.{
         .{ .Text = try textualBlockFromSlice(allocator, "response") },
-    });
+    }, null);
     conv.deinit();
 }
 
@@ -468,7 +464,7 @@ test "ContentBlock - Thinking variant" {
     try conv.addAssistantMessage(&.{
         .{ .Thinking = .{ .text = try textualBlockFromSlice(allocator, "hmm...") } },
         .{ .Text = try textualBlockFromSlice(allocator, "answer") },
-    });
+    }, null);
 
     try std.testing.expectEqual(@as(usize, 2), conv.messages.items[0].content.items.len);
     try std.testing.expectEqualStrings("hmm...", conv.messages.items[0].content.items[0].Thinking.text.items);
@@ -574,7 +570,7 @@ test "latestCompactionIndex - null when never compacted" {
     try conv.addUserMessage("hi");
     try conv.addAssistantMessage(&.{
         .{ .Text = try textualBlockFromSlice(allocator, "hello") },
-    });
+    }, null);
 
     try std.testing.expect(latestCompactionIndex(conv.messages.items) == null);
 }