feat: add configurable auto-simplify threshold and improve change detection

[kostyay]

• Add SimplifySettings interface with minChangedLines threshold, persisted in $PI_CODING_AGENT_DIR/simplify.json with sensible defaults (10 lines minimum)
• Implement readSettings() to load configuration with fallback to defaults when file is missing or malformed
• Add countChangedLines() to accurately measure added + removed lines using git diff --numstat and wc -l for untracked files
• Replace HEAD snapshot tracking with dirty file set snapshots for more reliable change detection — tracks which files were dirty at agent turn start
• Add wasAborted() helper to skip auto-simplify when agent run fails or is cancelled
• Update auto-simplify proposal logic to only trigger when changed lines exceed configured threshold and files are newly modified
• Refactor imports to use existsSync, extname, and join utilities for cleaner path/file handling
• Replace manual file extension parsing with extname() utility function
• Update README descriptions for simplify, status-bar, and sandbox extensions to reflect new capabilities