Plug the deep-embedder holes; remove public.zig escape hatch

Close the gap that forced the CLI to import internal libpanto namespaces,
then delete the transitional re-exports. The CLI now uses only the curated
public surface.

Additions to the public Agent facade:
- init/deinit (heap-pin the inner; the handle is a copyable value).
- addSystemMessage (.append) and setSystemPrompt (.replace).
- compact(override_system_prompt, extra) falling back to the config prompt.
- sessionId() accessor.

CompactionConfig gains compaction_prompt, which owns the compaction system
prompt for both auto-compaction and the explicit compact() default; the
Agent.compaction_system_prompt field is deleted. ConversationData is the
public name for the owned conversation value type (Session.load returns it,
Agent.init adopts it), distinct from the borrowed Conversation handle.

Agent.addSystemMessage/setSystemPrompt are kept on Agent (they persist with
the SystemMode) rather than dropped as the plan first proposed.

1 files changed, 12 insertions, 11 deletions

diff --git a/libpanto/src/agent.zig b/libpanto/src/agent.zig
index 5f9bee5..6d62b8a 100644
--- a/libpanto/src/agent.zig
+++ b/libpanto/src/agent.zig
@@ -206,11 +206,6 @@ pub const Agent = struct {
     /// Injectable streaming seam. Defaults to the real provider dispatch
     /// (`provider_mod.openStream`); tests override it with a stub.
     open_stream_fn: provider_mod.OpenStreamFn = provider_mod.openStream,
-    /// Compaction system prompt used for automatic compaction on context
-    /// overflow. Borrowed; set by the embedder (resolved from its
-    /// `COMPACTION.md` layers). When null, auto-compaction is disabled and
-    /// a context-overflow error propagates unchanged.
-    compaction_system_prompt: ?[]const u8 = null,
     /// Set by the embedder after `runStep` returns to learn whether an
     /// automatic compaction occurred this turn (so it can persist the
     /// rewritten conversation). Reset at the top of each `runStep`.
@@ -454,7 +449,7 @@ pub const Agent = struct {
         err: anyerror,
     ) !provider_mod.ProviderStream {
         if (self.auto_compacted) return err; // already retried once this turn
-        const sys = self.compaction_system_prompt orelse return err;
+        const sys = self.config.compaction.compaction_prompt orelse return err;
         const res = try self.compact(sys, null);
         if (!res.compacted) return err; // nothing to shed; give up
         self.auto_compacted = true;
@@ -595,11 +590,19 @@ pub const Agent = struct {
     /// if anything was compacted, appends the new compaction window
     /// (summary + restated kept suffix) to the store. Returns the same
     /// `CompactionResult` for the embedder to report.
+    ///
+    /// `override_system_prompt`, when non-null, is the compaction system
+    /// prompt for this run; when null it falls back to
+    /// `config.compaction.compaction_prompt`. With neither set, this errors
+    /// (`error.NoCompactionPrompt`).
     pub fn compactAndPersist(
         self: *Agent,
-        system_prompt: []const u8,
+        override_system_prompt: ?[]const u8,
         extra_instructions: ?[]const u8,
     ) !CompactionResult {
+        const system_prompt = override_system_prompt orelse
+            self.config.compaction.compaction_prompt orelse
+            return error.NoCompactionPrompt;
         const res = try self.compact(system_prompt, extra_instructions);
         if (res.compacted) {
             try turn_persist.persistCompaction(
@@ -2852,13 +2855,12 @@ test "runStep: auto-compacts on context overflow and retries once" {
     var h = TestHarness.init(allocator);
     defer h.deinit();
     h.activate();
-    h.config.compaction = .{ .keep_verbatim = 10 };
+    h.config.compaction = .{ .keep_verbatim = 10, .compaction_prompt = "Summarize the conversation." };
     var ns = null_store_mod.NullStore.init(allocator);
     var agent = Agent.init(allocator, io, &h.config, ns.store().create(), null);
     defer agent.deinit();
     h.seedInto(&agent);
     agent.open_stream_fn = stub.install();
-    agent.compaction_system_prompt = "Summarize the conversation.";
 
     const conv = &agent.conversation;
     try conv.addSystemMessage("you are helpful");
@@ -3233,13 +3235,12 @@ test "runStep: context-overflow compaction fires a compaction retry notification
     var h = TestHarness.init(allocator);
     defer h.deinit();
     h.activate();
-    h.config.compaction = .{ .keep_verbatim = 10 };
+    h.config.compaction = .{ .keep_verbatim = 10, .compaction_prompt = "Summarize the conversation." };
     var ns = null_store_mod.NullStore.init(allocator);
     var agent = Agent.init(allocator, io, &h.config, ns.store().create(), null);
     defer agent.deinit();
     h.seedInto(&agent);
     agent.open_stream_fn = stub.install();
-    agent.compaction_system_prompt = "Summarize the conversation.";
 
     const conv = &agent.conversation;
     try conv.addSystemMessage("you are helpful");