refactor(import): generalize import package across agents (Claude first)

Renames cmd/entire/cli/importclaude -> cmd/entire/cli/agentimport with an Importer seam (Discover + SplitTurns) and a static set of supported agents, so other agents can be added without touching the orchestration. Claude Code is the only implementation today.

Also drops the enable-time import prompt; importing is a hidden command only (entire import <agent>, discoverable via entire labs).