feat(pi-fff): add compact clickable rendering for search tools

## Summary

Add a compact TUI presentation for `ffgrep` and `fffind` in the Pi extension.

- Show a one-line tool header and one-line result preview by default.
- Click either part of a tool block to toggle the complete result.
- Keep `ffgrep` header options consistent: explicit `limit` and `context` render together as `(limit N, context N)`.
- Do not change search execution, tool arguments, or model-visible tool results.

## Verification

- `tsc --noEmit`
- `bun test test/extension.test.ts`

Start in the Pi extension code that renders ffgrep and fffind, then read test/extension.test.ts and run the existing tests. Verify that each tool has a one-line header and preview by default, clicking either toggles the full result, explicit limit and context appear together, and search behavior and model-visible results stay unchanged. Run tsc --noEmit and bun test test/extension.test.ts.

Written by the indexing model from the issue text.