pi-fff: switching modes leaves stale tool names active after `/reload`

Summary

registerPendingTools only ever adds names to the active tool set, never removes old ones. After switching from override back to a FFF-named mode and running /reload, the restored active list still contains the previous mode's tool names, so the model sees duplicate search tools.

Per the README, override replaces the built-in find / grep, and switching modes takes effect after /reload when the tools are registered again. The stale names contradict both statements.

Reproduction

1. Start Pi with pi-fff in override mode (grep / find active).
2. Run /fff-mode tools-and-ui, then /reload.
3. Check the active tools (e.g. via /tool or the model's tool list).

Actual

Stale search tools stay active alongside the final mode's names: grep, find, ffgrep, fffind (functionally duplicated in pairs).

Expected

The active set matches the final mode: only ffgrep / fffind (plus fff-multi-grep when enabled).

Verification (two-session mock on #854's branch)

Fresh extension instance per reload, one shared persisted active list:

direction	active set after reload	status
default -> override	read, grep, find	fixed by #854's prune
override -> default	read, grep, find, ffgrep, fffind	still open (this issue)

Notes

• The default-to-override direction was fixed as a side effect of #854's prune: pre-registered FFF names are instance-tracked, so they are removed from the active set.
• Fresh sessions are unaffected in either direction.
• A fix needs to distinguish the extension's grep / find registrations from Pi's built-ins sharing those names (e.g. via getAllTools source info), so it is kept out of #854.