pi-fff: Support enableHomeDirScanning / enableFsRootScanning to allow indexing home and root directories #588

Problem

The @ff-labs/fff-node FileFinder.create() API supports enableHomeDirScanning: true and enableFsRootScanning: true options to opt into indexing home directories (~) and the filesystem root (/). Without these flags, fff refuses to initialize:

"Can not run certain FFF features in a file system root or home directories. Consider smaller per-project directories."

However, the pi-fff extension (packages/pi-fff/src/index.ts) does not expose these options. The ensureFinder function always creates the finder with aiMode: true and no overrides, making it impossible to use fffind/ffgrep when pi is launched from ~ or /.

Proposed Solution

Expose enableHomeDirScanning and enableFsRootScanning to pi-fff users. Options:

1. Environment variables (e.g. FFF_ENABLE_HOME_SCAN=1, FFF_ENABLE_ROOT_SCAN=1) — simplest, matches the existing pattern (PI_FFF_MODE, FFF_FRECENCY_DB, etc.)
2. CLI flags (--fff-enable-home-scan, --fff-enable-root-scan) — consistent with existing flags
3. Both env vars and flags, following the existing precedence model

Environment

• pi-fff via pi install npm:@ff-labs/pi-fff
• macOS (Apple Silicon)
• Launching pi from ~ or /