add hunk-only exports for git diff modes

[hyle]

Summary

Add --hunks[=N] for --staged, --unstaged, and --diff so review-oriented exports can include only changed hunks plus configurable surrounding context.

Changes

• Add --hunks, --hunks=N, and --hunks N
• Reject --hunks outside explicit Git delta modes
• Add header metadata for hunk exports:
  • Hunks: on (context: N)
• Extend Git-selected path metadata with repo-relative current and previous paths for correct per-file hunk lookup
• Collect per-file unified hunk ranges from Git using path-scoped git diff -U0
• Keep added files as full-file exports under --hunks
• Treat zero-hunk delta entries as metadata-only:
  • keep them in Change Context
  • omit their file body
  • omit them from the project tree
• Add slice-aware render prep with renderer-owned visibility state
• Render omission markers between visible slices:
  • ... X unchanged lines omitted ...
• Preserve original file line numbers when --line-numbers is combined with --hunks
• Make tree rendering and byte/token metrics honor only render-visible entries
• Reject signed hunk context values such as --hunks=-1
• Update README docs and examples for hunk-mode behavior

Why

Git delta modes were already useful for narrowing scope, but they still exported full files. --hunks makes review exports substantially more precise without changing fuori’s model: a single Markdown artifact, grounded in Git, with exact metrics based on the final rendered output.

Verification

• Ran make test successfully
• Manual vetting of some output samples