refactor: revert lazy file loading and restore eager content reading

This reverts the architectural changes introduced in PR #2. While
technically valid, the lazy file reading pattern added unnecessary
mental overhead and complexity without a tangible benefit for our
current pipeline.
Since index.ts processes all scanned files anyway for chunking,
eagerly reading the file content during the initial [scanDirectory]
phase provides a simpler, more cohesive data contract (ScannedFile)
with no real-world performance penalty.