[codex] Fix monorepo file search fallback

[jamesrochabrun]

Summary

Fixes the Cmd+P quick file search regression where monorepos could always show no results after the optimized search path failed to produce a usable local index.

Root Cause

The git-backed local fallback could time out or return a partial list while indexing large repositories. When Spotlight had no usable result and the broad local index had no matching entries, Cmd+P had no query-specific rescue path and surfaced an empty result set.

Changes

• Drain git ls-files stdout reliably even when the process times out.
• Add a query-specific git file enumeration fallback when the broad local index has no matches.
• Keep ignored files filtered out while preserving tracked and untracked search hits.
• Add focused tests for partial monorepo index misses and real git query enumeration.

Validation

xcodebuild test -scheme AgentHubCore-Tests -destination 'platform=macOS' -test-timeouts-enabled YES -skipPackagePluginValidation -only-testing:AgentHubTests/FileIndexServicePrivacyTests -quiet

Exit code: 0.