Move session persistence into the Agent; collapse CLI plumbing

The Agent now owns its Conversation and a SessionStore, and persists
everything it generates as turns progress. Embedders get persistence for
free; the CLI no longer drives the session log.

libpanto:
- Agent.init takes a SessionStore + optional Conversation to adopt (resume
  path). Agent owns/tears down the conversation; turn-driving methods drop
  the *Conversation param and operate on self.conversation.
- New Agent methods: submitUserMessage (adds+persists the user prompt
  immediately), addSystemMessage(text, mode), runStep persists the turn on
  every exit path (incl. partial turns on error), compactAndPersist for the
  explicit /compact path. Auto-compaction persists its window via runStep's
  tail.
- turn_persist.zig: DiskMessage mapping moved out of the CLI; reads usage
  off Message.usage (no per-message-usage list). System mode rides on
  DiskMessage.mode, derived from the System block.
- NullStore is now an allocator-bearing struct (frees consumed messages per
  the store-consumes-messages contract).
- Tests: in-memory CapturingStore round-trip + NullStore turn test.

panto CLI:
- Delete src/session_persist.zig. REPL is submitUserMessage + runStep.
- Reorder construction: store -> registry -> agent -> bootstrap -> seed/
  reconcile system prompt through the agent -> load extensions.
- command.Context drops conv/session_mgr; commands reach ctx.agent.
- system_prompt seed/reconcile call agent.addSystemMessage.
- CLIReceiver is display-only (per_message_usage removed).

Phases 3-5 of docs/pluggable-session-store.md. Behavior preserved; full
build + test suite green.

1 files changed, 1 insertions, 9 deletions

diff --git a/src/compaction.zig b/src/compaction.zig
index a1f6e89..4ec5732 100644
--- a/src/compaction.zig
+++ b/src/compaction.zig
@@ -8,7 +8,6 @@
 const std = @import("std");
 const panto = @import("panto");
 const command = @import("command.zig");
-const session_persist = @import("session_persist.zig");
 
 /// Install the `/compact` command into `registry`.
 pub fn register(registry: *command.Registry) !void {
@@ -23,20 +22,13 @@ pub fn register(registry: *command.Registry) !void {
 fn run(args: []const u8, ctx: *command.Context) anyerror!void {
     const extra: ?[]const u8 = if (args.len > 0) args else null;
 
-    const res = ctx.agent.compact(ctx.conv, ctx.compaction_prompt, extra) catch |err| {
+    const res = ctx.agent.compactAndPersist(ctx.compaction_prompt, extra) catch |err| {
         try ctx.stdout.print("\n[compaction failed: {s}]\n> ", .{@errorName(err)});
         try ctx.stdout_file.flush();
         return;
     };
 
     if (res.compacted) {
-        try session_persist.persistCompaction(
-            ctx.allocator,
-            ctx.session_mgr,
-            ctx.conv,
-            ctx.provider_name,
-            ctx.model_name,
-        );
         try ctx.stdout.print(
             "\n[compacted: summarized {d} message(s), kept {d} recent turn(s)]\n> ",
             .{ res.summarized_messages, res.kept_turns },