feat(enable): offer to import agent history on first-time enable

First-time entire enable now offers to import pre-existing agent history for the agents selected during setup. Interactive runs show an unchecked multi-select of eligible agents — those with a registered importer and at least one discoverable session in the 30-day lookback window. Non-interactive runs (--yes or no TTY) auto-import all eligible agents. The offer is gated to first run via IsSetUpAny and is best-effort: discovery or import failures are logged and surfaced as a notice but never fail enable.

Wired into both first-run enable paths — runEnableInteractive (bare / --yes) and setupAgentHooksNonInteractive (--agent) — reusing the same redaction + agentimport.Run flow as the standalone entire import.