Append-only log.md

The 0.1 version of the specification calls for having elements in log.md ordered newest-first.

That's great for a human opening the file and wanting to see the latest activity at the top, but these files are expected to grow over time, and will be read and written to by agents using tools.

With the newest-first order, Agents must use some kind of edit tool to add a new element at the top of the file, and it can become prohibitively expensive as the log file grows. Sometimes, the agent will also resort to temporary files or weird bits of scripts to add to the file in the proper order.

Doing it the other way and adding new elements at the bottom of the file allows the agent to just do a `cat >>` or some other append method, which is much more efficient.

The spec should at least allow, if not encourage, an append-only format.