Add SessionTitle service that generates a session title from the
session's sole user message via the customizable "title" agent, then
renames the session. Runs once per session, gated on durable history
having exactly one user message (no default-title string comparison).
Wire it into SessionRunner as a background fork after the first turn's
prompt promotion makes the user message visible; forking before
promotion caused the first-message lookup to see zero rows and silently
no-op.
Closes#34364
Adds a plugin.list HTTP endpoint that reports currently loaded plugins, wires it through protocol, server, and generated clients, and merges it into sdk-next's plugin API. Also updates the sample plugin to register a primary agent instead of a subagent for local testing.