docs: document reporting directory concept and descent algorithm in README

[Copilot]

The README described group mode descent only vaguely ("descends until a directory containing at least one file is found") with no explanation of what a reporting directory is or how the ignore list interacts with descent.

Changes

• New "Reporting Directory & Descent" section between "Operating Modes" and "Example Output" covering:
  • Definition of a reporting directory (first ancestor containing a visible, non-ignored file)
  • Step-by-step descent rules: stop on visible file → recurse on ignored-only/subdir-only → use empty leaf as-is → fall back to current dir if all subdirs are inaccessible
  • Ignore list semantics during descent: matched files are invisible (don't halt descent); matched subdirectories are skipped entirely and never become reporting directories
  • Concrete example clarifying that ['.*', '*.tmp'] causes ontrack to look past hidden dirs and dotfile-only directories