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.

0 files changed, 0 insertions, 0 deletions