ai-agent: carry the head round's counts on a conversation row (#173)

A Session Flow round's header counts what the conversation did as of that
round: distinct workspace change records with the lines their diffs add and
remove, provider calls, child agents started, and shell commands run through
the runtime's Bash tool. ConversationRow now carries them as changes,
linesAdded, linesRemoved, llmCalls, subagents and bashRuns, each absent rather
than zero when the round did not carry it, so a list can show them without
folding a conversation and a Sessionizer older than a count still lists.