Add pluggable SessionStore interface with FSJSONL + Null backends

Introduce a neutral persistence seam (session_store.zig) following the
{ptr, vtable} shape of the other libpanto seams. The interface traffics in
DiskMessage so non-JSONL backends (e.g. Postgres) can implement it.

- session_store.zig: SessionStore vtable (appendMessages, loadConversation,
  sessionId, activeModel), LoadedSession{conversation, dangling_user},
  re-exported disk types, and an FSJSONLStore alias.
- session_manager.zig: add store() wrapper + loadConversation() with
  dangling trailing-user detection (excluded from the rebuilt conversation,
  returned as dangling_user).
- null_store.zig: no-op backend, stateless singleton.
- root.zig: export session_store + null_store.

Phase 1+2 of docs/pluggable-session-store.md. Behavior-preserving; CLI not
yet rewired.

1 files changed, 2 insertions, 0 deletions

diff --git a/libpanto/src/root.zig b/libpanto/src/root.zig
index 3f5549c..39797dd 100644
--- a/libpanto/src/root.zig
+++ b/libpanto/src/root.zig
@@ -10,6 +10,8 @@ pub const tool_source = @import("tool_source.zig");
 pub const tool_registry = @import("tool_registry.zig");
 pub const session = @import("session.zig");
 pub const session_manager = @import("session_manager.zig");
+pub const session_store = @import("session_store.zig");
+pub const null_store = @import("null_store.zig");
 pub const pricing = @import("pricing.zig");
 pub const compaction = @import("compaction.zig");
 pub const image = @import("image.zig");