feat(core): generate session titles from first prompt

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