[Optimization] Streamline fffind & ffgrep promptGuidelines into concise heuristics

Summary

@ff-labs/pi-fff currently injects 10 separate promptGuidelines entries (~800 characters) across fffind and ffgrep into Pi's System Prompt on every session.

Suggestion

Many of the 10 bullets describe overlapping usage heuristics (e.g., bare identifiers, path include/exclude, glob usage, 1-2 term queries). These can easily be consolidated into 2 concise summary bullets:

1. ffgrep: prefer bare identifiers, literal queries, and path scoping (e.g., path: 'src/', exclude: 'test/').
2. fffind: whole-path fuzzy search (1-2 terms); use path glob (e.g., path: '**/file.h') for exact matches.

This keeps the guidance clear and effective while cutting the token footprint in half across all user sessions.