fix(pi-fff): preserve FFF tool renderers for historical calls after `/reload`

Summary

pi-fff registers its tools in session_start, but Pi restores historical tool rows before it emits that event. After /reload, existing ffgrep and fffind rows therefore cannot resolve their custom renderCall / renderResult functions and fall back to the generic tool display. Calls made after the reload render normally.

This predates #851. #851 makes the problem more visible because its compact renderer is lost after reload, but does not introduce the lifecycle mismatch.

Reproduction

1. Start Pi with pi-fff in its default FFF-named mode.
2. Call ffgrep or fffind.
3. Run /reload.
4. Inspect the pre-reload tool row.

Actual

The historical row displays the tool name, JSON arguments, and raw output through Pi's generic fallback.

Expected

The historical row uses the same pi-fff renderer as a newly created row in the same session.

Screenshot

Scope

A fix can register the non-builtin FFF-named definitions during extension loading so historical rows can resolve their renderers. It must leave final tool activation, mode restoration, search execution, arguments, and model-visible results in session_start.

Historical override-mode grep / find rows are outside this narrow fix: registering those names before mode restoration would override Pi's built-in names too early.