harness: live tool-call bubble during agent turns (one message, m.replace edits, capped at 10 lines/3.5k chars, final reply as fresh message)

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Co-Authored-By: Letta Code <noreply@letta.com>

src/matrix.zig

@@ -354,6 +354,30 @@ pub fn sendText(alloc: std.mem.Allocator, io: Io, token: []const u8, room: []con
     return ev.val;
 }
 
+/// Edit an existing m.text message in place (m.replace). `orig_event_id` is
+/// the event being replaced; `new_body` is the full replacement text. The
+/// plain-text `body` carries the "* " legacy-edit prefix for old clients.
+pub fn editText(alloc: std.mem.Allocator, io: Io, token: []const u8, room: []const u8, orig_event_id: []const u8, new_body: []const u8) ![]u8 {
+    const ts = Io.Clock.now(.real, io).nanoseconds;
+    const url = try std.fmt.allocPrint(alloc, "{s}/_matrix/client/v3/rooms/{s}/send/m.room.message/infr-edit-{d}", .{ HOMESERVER, room, ts });
+    defer alloc.free(url);
+
+    var payload: std.ArrayList(u8) = .empty;
+    defer payload.deinit(alloc);
+    try payload.appendSlice(alloc, "{\"msgtype\":\"m.text\",\"body\":\"* ");
+    try jsonEscape(alloc, &payload, new_body);
+    try payload.appendSlice(alloc, "\",\"m.new_content\":{\"msgtype\":\"m.text\",\"body\":\"");
+    try jsonEscape(alloc, &payload, new_body);
+    try payload.appendSlice(alloc, "\"},\"m.relates_to\":{\"rel_type\":\"m.replace\",\"event_id\":\"");
+    try jsonEscape(alloc, &payload, orig_event_id);
+    try payload.appendSlice(alloc, "\"}}");
+
+    const resp = try http(alloc, io, "PUT", url, token, payload.items);
+    defer alloc.free(resp);
+    const ev = letta.jsonStr(alloc, resp, 0, "event_id") orelse return error.SendFailed;
+    return ev.val;
+}
+
 /// Join a room by id (accepts invites).
 pub fn joinRoom(alloc: std.mem.Allocator, io: Io, token: []const u8, room: []const u8) !void {
     const url = try std.fmt.allocPrint(alloc, "{s}/_matrix/client/v3/rooms/{s}/join", .{ HOMESERVER, room });