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.

1 files changed, 19 insertions, 25 deletions

diff --git a/libpanto/src/public.zig b/libpanto/src/public.zig
index 31d5f7b..8a6fcd1 100644
--- a/libpanto/src/public.zig
+++ b/libpanto/src/public.zig
@@ -127,32 +127,24 @@ pub const Event = stream_mod.Event;
 pub const ContentBlockType = provider_mod.ContentBlockType;
 pub const ProviderRetryInfo = provider_mod.ProviderRetryInfo;
 
-/// The transparent turn state machine, renamed from the internal `Phase`.
-pub const State = agent_mod.Stream.Phase;
+/// The pull event stream for one turn. Aliased straight through: its public
+/// surface is exactly `next`/`deinit` plus the transparent `state` field
+/// (`State`); every other field is underscore-prefixed internal state.
+/// `Agent.run` returns a `*Stream` (heap-pinned); the caller owns it and
+/// must `deinit` it (which persists the turn tail and frees the allocation).
+///
+/// `next()` returns `!?Event`: a value is progress (including the terminal
+/// event), `null` is exhaustion, an error is a genuine failure.
+pub const Stream = agent_mod.Stream;
 
-pub const Stream = struct {
-    inner: *agent_mod.Stream,
-
-    /// `!?Event`: a value is progress (including the terminal event), `null`
-    /// is exhaustion, an error is a genuine failure.
-    pub fn next(self: Stream) !?Event {
-        return self.inner.next();
-    }
-    pub fn deinit(self: Stream) void {
-        // The internal `Stream.deinit` persists the turn tail and frees its
-        // own heap allocation.
-        self.inner.deinit();
-    }
-    pub fn state(self: Stream) State {
-        return self.inner.phase;
-    }
-};
+/// The transparent turn state machine (`Stream.state`).
+pub const State = agent_mod.Stream.State;
 
 // ===========================================================================
 // Agent (behavioral, pointer-wrapped)
 // ===========================================================================
 
-pub const UserMessage = struct { text: []const u8 };
+pub const UserMessage = agent_mod.UserMessage;
 pub const CompactionResult = agent_mod.Agent.CompactionResult;
 
 pub const Agent = struct {
@@ -192,20 +184,22 @@ pub const Agent = struct {
     pub fn setConfig(self: Agent, cfg: *const Config) void {
         self.inner.setConfig(cfg);
     }
-    pub fn run(self: Agent, message: UserMessage) !Stream {
-        return .{ .inner = try self.inner.run(.{ .text = message.text }) };
+    /// Submit a user message and begin a turn, returning a heap-pinned
+    /// `*Stream` the caller owns and must `deinit`.
+    pub fn run(self: Agent, message: UserMessage) !*Stream {
+        return self.inner.run(message);
     }
 
     /// Append a system message to the conversation and persist it (the
     /// `.append` `SystemMode`: it adds to the effective prompt).
     pub fn addSystemMessage(self: Agent, text: []const u8) !void {
-        return self.inner.addSystemMessage(text, .append);
+        return self.inner.addSystemMessage(text);
     }
     /// Replace the effective system prompt and persist it (the `.replace`
     /// `SystemMode`: it discards all prior system text). On replay the log
     /// reconstructs the same effective prompt.
     pub fn setSystemPrompt(self: Agent, text: []const u8) !void {
-        return self.inner.addSystemMessage(text, .replace);
+        return self.inner.setSystemPrompt(text);
     }
 
     /// Compact and persist (the explicit `/compact` entry point).
@@ -214,7 +208,7 @@ pub const Agent = struct {
     /// to the compaction prompt for this run (the `/compact $ARGUMENTS`
     /// path).
     pub fn compact(self: Agent, override_system_prompt: ?[]const u8, extra: ?[]const u8) !CompactionResult {
-        return self.inner.compactAndPersist(override_system_prompt, extra);
+        return self.inner.compact(override_system_prompt, extra);
     }
 
     /// A borrowed pointer to the agent's live conversation, for in-place