Incremental scan — only scan changed files

[ajianaz]

Description

For large codebases, scanning every file is slow and expensive. Implement incremental scanning that only processes files that changed since the last review.

Acceptance Criteria

• Track file state (hash) from previous reviews in a local cache (.cora-cache)
• cora scan --incremental only reviews changed files
• cora scan --incremental --base-commit HEAD~5 compares against a specific commit
• Cache is gitignore-friendly and portable
• Show summary of files skipped vs reviewed

Implementation Notes

• Use git diff --name-only for git-aware incremental scans
• Fall back to mtime/hash comparison in non-git directories
• Store cache in .cora-cache.json in project root

[ajianaz]

Implemented in PR #24, merged to develop.