Centralize repository root and output directory helpers

[rosspeili]

Problem

Paths to outputs/, system_sounds/, assets/enrollment.jpg, and room CSVs are built ad hoc (os.path.dirname(__file__), joins in multiple modules). This complicates relocation, packaging, and Windows vs. WSL paths.

Goal

Small internal module or functions: repo_root(), outputs_subdir(name), used by mail.py, room.py, image save in generate_image_from_prompt, splash audio, etc.

Acceptance criteria

• Single source of truth for repo root detection.
• README / docs/CONFIGURATION.md updated if env or layout changes.
• No change to default on-disk layout unless intentional.