refactor: centralize shared helper utilities

[fmueller]

Summary

• add scribae.common as a single home for shared helper primitives (Reporter, slugify, report, current_timestamp)
• remove duplicated helper implementations from idea/brief/meta/write/refine/feedback/language and switch those modules to shared imports
• keep behavior stable while reducing duplication and making helper updates one-touch
• add focused unit tests for shared helpers in tests/unit/common_test.py

Validation

• uv run ruff check
• uv run mypy
• uv run pytest