libpanto API fixes

- FSJSONLStore: don't require `cwd`, instead panto CLI provides it in
  the session metadata
- Lua: expose the `SessionStore` interface and the `FSJSONLStore` and
  `NullStore` implementations
- C: use idiomatic `const char *` for incoming strings
- C: expose the `FSJSONLStore` and `NullStore` implementations

1 files changed, 26 insertions, 9 deletions

diff --git a/libpanto/src/session.zig b/libpanto/src/session.zig
index 492f94b..843989a 100644
--- a/libpanto/src/session.zig
+++ b/libpanto/src/session.zig
@@ -87,12 +87,14 @@ pub const SessionHeader = struct {
     version: u32,
     id: []const u8, // UUIDv7 string, owned
     timestamp: []const u8, // ISO 8601, owned
-    cwd: []const u8, // owned
+    /// Opaque session-wide metadata bag. Round-trips verbatim; `libpanto`
+    /// never interprets it. The panto CLI records `{ "cwd": ... }` here.
+    metadata: ?[]const u8 = null, // owned
 
     pub fn deinit(self: SessionHeader, alloc: Allocator) void {
         alloc.free(self.id);
         alloc.free(self.timestamp);
-        alloc.free(self.cwd);
+        if (self.metadata) |m| alloc.free(m);
     }
 };
 
@@ -308,8 +310,12 @@ pub fn serializeHeader(allocator: Allocator, header: SessionHeader) ![]u8 {
     try s.write(header.id);
     try s.objectField("timestamp");
     try s.write(header.timestamp);
-    try s.objectField("cwd");
-    try s.write(header.cwd);
+    if (header.metadata) |md| {
+        var parsed = try std.json.parseFromSlice(std.json.Value, allocator, md, .{});
+        defer parsed.deinit();
+        try s.objectField("metadata");
+        try s.write(parsed.value);
+    }
     try s.endObject();
 
     return try aw.toOwnedSlice();
@@ -568,13 +574,24 @@ fn parseHeaderFromObject(allocator: Allocator, obj: std.json.ObjectMap) ParseErr
     errdefer allocator.free(id);
     const timestamp = try dupeStringField(allocator, obj, "timestamp");
     errdefer allocator.free(timestamp);
-    const cwd = try dupeStringField(allocator, obj, "cwd");
-    errdefer allocator.free(cwd);
+    const metadata: ?[]const u8 = blk: {
+        if (obj.get("metadata")) |mv| {
+            break :blk try std.json.Stringify.valueAlloc(allocator, mv, .{});
+        }
+        if (obj.get("cwd")) |cv| {
+            if (cv != .string) return error.MissingField;
+            const cwd_json = try std.json.Stringify.valueAlloc(allocator, cv, .{});
+            defer allocator.free(cwd_json);
+            break :blk try std.fmt.allocPrint(allocator, "{{\"cwd\":{s}}}", .{cwd_json});
+        }
+        break :blk null;
+    };
+    errdefer if (metadata) |m| allocator.free(m);
     return .{
         .version = version,
         .id = id,
         .timestamp = timestamp,
-        .cwd = cwd,
+        .metadata = metadata,
     };
 }
 
@@ -956,7 +973,7 @@ test "serialize/parse header round-trip" {
         .version = 1,
         .id = try dupe(a, "019dc5ba-53f6-71a5-ab8f-b1f8709c2572"),
         .timestamp = try dupe(a, "2026-04-25T17:40:15.990Z"),
-        .cwd = try dupe(a, "/Users/travis/Code/pantograph"),
+        .metadata = try dupe(a, "{\"cwd\":\"/Users/travis/Code/pantograph\"}"),
     };
     defer header.deinit(a);
 
@@ -968,7 +985,7 @@ test "serialize/parse header round-trip" {
     try testing.expect(fe == .header);
     try testing.expectEqual(@as(u32, 1), fe.header.version);
     try testing.expectEqualStrings(header.id, fe.header.id);
-    try testing.expectEqualStrings(header.cwd, fe.header.cwd);
+    try testing.expectEqualStrings(header.metadata.?, fe.header.metadata.?);
 }
 
 test "serialize/parse user message entry round-trip (with provider/model stamp)" {