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, 3 insertions, 5 deletions

diff --git a/src/command.zig b/src/command.zig
index 26acc7a..49ad5b8 100644
--- a/src/command.zig
+++ b/src/command.zig
@@ -32,12 +32,10 @@ const panto = @import("panto");
 pub const Context = struct {
     allocator: std.mem.Allocator,
 
-    /// In-memory conversation the agent is driving.
-    conv: *panto.conversation.Conversation,
-    /// The active agent (model/provider/tooling snapshot).
+    /// 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,
-    /// Session log to persist any changes a command makes.
-    session_mgr: *panto.session_manager.SessionManager,
 
     /// REPL output writer and its backing file writer (for flushing).
     stdout: *std.Io.Writer,