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, 4 insertions, 4 deletions

diff --git a/src/command.zig b/src/command.zig
index 49ad5b8..0f6fb01 100644
--- a/src/command.zig
+++ b/src/command.zig
@@ -32,10 +32,10 @@ const panto = @import("panto");
 pub const Context = struct {
     allocator: std.mem.Allocator,
 
-    /// The active agent. Owns the conversation (`agent.conversation`) and
-    /// the session store (`agent.session_store`); commands reach both
-    /// through it rather than holding separate pointers.
-    agent: *panto.agent.Agent,
+    /// The active agent (a copyable public handle). Commands reach the
+    /// conversation via `agent.conversation()` and compact via
+    /// `agent.compact(...)`.
+    agent: panto.Agent,
 
     /// REPL output writer and its backing file writer (for flushing).
     stdout: *std.Io.Writer,