Sync Agent/Stream internals to public names; alias Stream

Push the API-shaping we'd done in the public facade down into the internal
types, collapsing the wrappers:

- Stream: rename phase->state, Phase->State; underscore-prefix the internal
  fields (_agent/_queue/_response/_start/_persisted/_pending_error). state
  is the one intended-public field. public.zig now aliases Stream; run
  returns *Stream.
- Agent compaction: pure transform compact()->private _compactInPlace;
  compactAndPersist()->compact() (the public name, persists).
- Agent system prompt: addSystemMessage(text,mode) split into
  addSystemMessage(text) + setSystemPrompt(text) over a private
  _persistSystemMessage.
- UserMessage moved off Agent to module scope.
- Underscore-prefix pure-internal Agent fields: _open_stream_fn,
  _auto_compacted, _retry_prng.

The Agent facade is now pure 1:1 forwarders; it stays a wrapper only because
init heap-pins the inner (copyable, move-safe handle) and conversation()/
sessionId() are accessors. Conversation and Stream are plain aliases.

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