Forked background sessions get an explicit worktree warning injected into transcript

When spawning a background or forked session with keepParent, a synthetic "fork_briefing" message is now inserted directly into the new session's transcript, not just the system prompt. It warns the forked session never to edit files in the original session's worktree and to create its own instead, referencing the parent's worktreePath and worktreeBranch.